Expose zpool guids through procfs

[anodos325]

There are various places in middleware where we
look up zpool status and guid. Since the guid
is readily available through spa_guid() providing
via procfs is a significant enhancement because
it will reduce allow us to eliminate some frequent
libzfs / py-libzfs calls in favor of just reading
via procfs.

Signed-off-by: Andrew Walker awalker@ixsystems.com

Motivation and Context

Description

How Has This Been Tested?

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.

[anodos325]

Need to add dealloc. Also appears that pool state not visible on FreeBSD side (possible bug).

[ghost]

There is a build error in the CI:

 /tmp/zfs-build-runner-Wj2VMpjj/BUILD/zfs-kmod-2.1.4/_kmod_build_5.13.0-1022-azure/../zfs-2.1.4/module/zfs/spa_stats.c: In function 'spa_guid_data':
/tmp/zfs-build-runner-Wj2VMpjj/BUILD/zfs-kmod-2.1.4/_kmod_build_5.13.0-1022-azure/../zfs-2.1.4/module/zfs/spa_stats.c:826:32: error: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'uint64_t' {aka 'long long unsigned int'} [-Werror=format=]
  826 |  (void) snprintf(buf, size, "%lu\n", spa_guid(spa));
      |                              ~~^     ~~~~~~~~~~~~~
      |                                |     |
      |                                |     uint64_t {aka long long unsigned int}
      |                                long unsigned int
      |                              %llu
cc1: all warnings being treated as errors

Linux is a bit weird about this so in ZFS we use %llu and also cast the value to u_longlong_t when printing 64-bit uints.

[ghost]

Please sign-off the commit when you update so we can see checkstyle pass, too.