xfs: hopefully the last few rt fixes

Here's the last couple of rt fixes for 5.10.  The first one fixes a
lockdep complaint, and the second one was me realizing I made a major
thinko in an earlier patch: not realizing that xfs_free_file_space
is really a "make contents zero by punching or manually zeroing as
needed" function; the solution is to push all the rt extent alignment
checking up to xfs_file_fallocate.

Updated test case attached.