libceph: use min() to simplify code in ceph_dns_resolve_name()

When resolving name in ceph_dns_resolve_name(), the end address of name
is determined by the minimum value of delim_p and colon_p. So using min()
here is more in line with the context.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>