gfs2: Don't get stuck writing page onto itself under direct I/O

When a direct I/O write is performed, iomap_dio_rw() invalidates the
part of the page cache which the write is going to before carrying out
the write.  In the odd case, the direct I/O write will be reading from
the same page it is writing to.  gfs2 carries out writes with page
faults disabled, so it should have been obvious that this page
invalidation can cause iomap_dio_rw() to never make any progress.
Currently, gfs2 will end up in an endless retry loop in
gfs2_file_direct_write() instead, though.

Break this endless loop by limiting the number of retries and falling
back to buffered I/O after that.

Also simplify should_fault_in_pages() sightly and add a comment to make
the above case easier to understand.

Reported-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>