libceph: introduce alloc_watch_request()

ceph_osdc_alloc_messages() call will be moved out of
alloc_linger_request() in the next commit, which means that
ceph_osdc_watch() will need to call ceph_osdc_alloc_messages()
twice.  Add a helper for that.

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