gfs2: Use mapping->gfp_mask for metadata inodes
Set mapping->gfp mask to GFP_NOFS for all metadata inodes so that
allocating pages in the address space of those inodes won't call back
into the filesystem. This allows to switch back from
find_or_create_page() to grab_cache_page() in two places.
Partially reverts commit
220cca2a4f58 ("GFS2: Change truncate page
allocation to be GFP_NOFS").
Thanks to Dan Carpenter <dan.carpenter@linaro.org> for pointing out a
Smatch static checker warning.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>