gfs2: Use iomap for stuffed direct I/O reads

Remove the fallback code from direct to buffered I/O for stuffed reads.

For stuffed writes, we must keep the fallback code: the deferred glock
we are holding under direct I/O doesn't allow to write to the inode or
change the file size.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>