Use <fcntl.h> instead of <sys/fcntl.h>

[thesamesam]

Motivation and Context

<sys/fcntl.h> is deprecated and triggers a warning (-> error) on musl.

Bug: https://bugs.gentoo.org/925235

Description

When building on musl, we get:

In file included from tests/zfs-tests/cmd/getversion.c:22:
/usr/include/sys/fcntl.h:1:2: error: #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h> [-Werror=cpp]
    1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>

In file included from module/os/linux/zfs/vdev_file.c:36:
/usr/include/sys/fcntl.h:1:2: error: #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h> [-Werror=cpp]
    1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>                                                                                                                     

How Has This Been Tested?

Tested on musl and glibc on amd64.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

@thesamesam is there any reason this is still marked as a draft?

[thesamesam]

Completely forgot. I've got a busy few days but I'll put this on my list to come back to.

I need to double check if there's any other missing includes and whether they need a guard - there was one I dropped which it turns out is needed for the kernel build but is wrong for the userland build, or something like that.

[behlendorf]

@thesamesam if this is still needed can you rebase this and mark it ready for review.

[thesamesam]

The snag when I last looked was something about getting the include right for the module vs the userland side, I'll look on the weekend - sorry for keeping you waiting.

[behlendorf]

Makes sense. Can you just remove the draft status from this PR and update the commit comment to address the make checkstyle warnings.

[thesamesam]

Whoops -- something about this PR is cursed :)

Thank you for your patience again. I'm sorry for being a snail on this -- my only musl system wasn't very accessible and I ended up making some bad assumptions about something else I'd seen when it turns out it was unrelated.