gfs2: Prevent endless loops in gfs2_file_buffered_write

Currently, instead of performing a short write,
iomap_file_buffered_write will fail when part of its iov iterator cannot
be read.  In contrast, gfs2_file_buffered_write will loop around if it
can read part of the iov iterator, so we can end up in an endless loop.

This should be fixed in iomap_file_buffered_write (and also
generic_perform_write), but this comes a bit late in the 5.16
development cycle, so work around it in the filesystem by
trimming the iov iterator to the known-good size for now.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>