xfs: XFS_IOCTL_SETXATTR can run in user namespaces

Currently XFS_IOCTL_SETXATTR will fail if run in a user namespace as
it it not allowed to change project IDs. The current code, however,
also prevents any other change being made as well, so things like
extent size hints cannot be set in user namespaces. This is wrong,
so only disallow access to project IDs and related flags from inside
the init namespace.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>