ceph: convert int fields in ceph_mount_options to unsigned int

Most of these values should never be negative, so convert them to
unsigned values. Add some sanity checking to the parsed values, and
clean up some unneeded casts.

Note that while caps_max should never be negative, this patch leaves
it signed, since this value ends up later being compared to a signed
counter. Just ensure that userland never passes in a negative value
for caps_max.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>