gfs2: Pass glock holder to gfs2_file_direct_{read,write}

Pass a pointer to the existing glock holder from
gfs2_file_{read,write}_iter to gfs2_file_direct_{read,write}
to save some stack space.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>