gfs2: Fix and clean up {GET,SET}FLAGS ioctl

Switch to a simple array for mapping between the FS_*_FL and GFS_DIF_*
flags.  Clarify how the mapping between FS_JOURNAL_DATA_FL and the
filesystem flags works.  The GFS2_DIF_SYSTEM flag cannot be set from
user space, so remove it from GFS2_FLAGS_USER_SET.  Fail with -EINVAL
when trying to set flags that are not supported instead of silently
ignoring those flags.

Partially fixes xfstest generic/424.

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