Retire .write/.read file operations

[behlendorf]

The .write/.read file operations callbacks can be retired since
support for .read_iter/.write_iter and .aio_read/.aio_write has
been added. The vfs_write()/vfs_read() entry functions will
select the correct interface for the kernel. This is desirable
because all VFS write/read operations now rely on common code.

This change also add the generic write checks to make sure that
ulimits are enforced correctly on write. A similar check was
added to zpl_fallocate() so the truncate case is also handled
properly.

• 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)

[mention-bot]

@behlendorf, thanks for your PR! By analyzing the history of the files in this pull request, we identified @ryao, @tuxoko and @dweeezil to be potential reviewers.

[tuxoko]

Unfortunately the prototype of generic_write_check is changed when aio_write is retired in kernel. However, currently we use iter_write when it exists, which is introduced before that. So this won't compile between the time iter_write introduced and aio_write retired

[behlendorf]

Not surprisingly it looks like we're going to need a wrapper function and configure check for generic_write_checks().

[behlendorf]

@tuxoko I'll see about adding some compatibility code for the prototype early next week assuming you're OK with the overall approach taken in this patch.

[tuxoko]

@behlendorf
Yeah, but you should also add generic_segment_checks to aio_read

[behlendorf]

I'll add it when I next refresh this.

[behlendorf]

Superseded by #5673.