xfs: introduce xfs_quota_defs.h

There are a lot of quota flag definitions that are shared by user
and kernel space. Move them all to xfs_quota_defs.h so we can
unshare xfs_quota.h and remove the __KERNEL__ regions from it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>