Linux 5.10 compat: restore custom uio_prefaultpages()

[behlendorf]

Motivation and Context

Issue #11463. Regression observed on CentOS 7.9 due to Linux 5.10
compatibility changes.

Description

As part of commit 1c2358c the custom uio_prefaultpages() code
was removed in favor of using the generic kernel provided
iov_iter_fault_in_readable() interface. Unfortunately, it
turns out that up until the Linux 4.7 kernel the function would
only ever fault in the first iovec of the iov_iter. The result
being uiomove_iov() may hang waiting for the page.

This commit effectively restores the custom uio_prefaultpages()
pages code for Linux 4.9 and earlier kernels which contain the
troublesome version of iov_iter_fault_in_readable().

How Has This Been Tested?

Using the Postgres reproducer provided in issue #11463. Without
the patch applied I'm able to easily reproduce the issue. With the
change applied I can create the database on ZFS without any issue.

$ sudo -u postgres psql -c 'create database test;'

Note: I still need to confirm that there isn't an issue on 4.9 and newer kernels.
I don't expect there to be an any issue, but it needs to be verified. In the
meanwhile I wanted to get the PR open for testing.

I've verified that the postgres test case works correctly for me on a
Fedora 5.10.8-200.fc33.x86_64 kernel both with and without the patch
applied. This was as expected.

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.