libceph: change ceph_pagelist_encode_string() to take u32

The wire format dictates that the length of string fits into 4 bytes.
Take u32 instead of size_t to reflect that.

We were already truncating len in ceph_pagelist_encode_32() -- this
just pushes that truncation one level up.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>