ceph: make fsc->mount_state an int

This field is an unsigned long currently, which is a bit of a waste on
most arches since this just holds an enum. Make it (signed) int instead.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>