xfs: separate xflags from xfs_ioctl_setattr

The setting of the extended flags is down through two separate
interfaces, but they are munged together into xfs_ioctl_setattr
and make that function far more complex than it needs to be.
Separate it out into a helper function along with all the other
common inode changes and transaction manipulations in
xfs_ioctl_setattr().

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