Fix O_APPEND for Linux 3.15 and older kernels - 2.1 backport

[behlendorf]

Motivation and Context

Backport of #13377

Description

When using a Linux kernel which predates the iov_iter interface the
O_APPEND flag should be applied in zpl_aio_write() via the call to
generic_write_checks(). The updated pos variable was incorrectly
ignored resulting in the current offset being used.

This issue should only realistically impact the RHEL/CentOS 7.x
kernels which are based on Linux 3.10.

How Has This Been Tested?

See #13377

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.