gfs2: Always mark inode dirty in fallocate

When allocating space with fallocate, always update the file timestamps
and mark the inode dirty, no matter if the FALLOC_FL_KEEP_SIZE flag is
set or not.  The inode needs to be marked dirty so that a subsequent
fsync will pick it up and any new allocations will make it to disk.
Filesystems like xfs and ext4 always update the timestamps, so make
gfs2 behave the same way.

Fixes xfstest generic/483.

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