cifs: Make wait_mtu_credits take size_t args

Make the wait_mtu_credits functions use size_t for the size and num
arguments rather than unsigned int as netfslib uses size_t/ssize_t for
arguments and return values to allow for extra capacity.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: linux-cachefs@redhat.com
cc: netfs@lists.linux.dev
cc: linux-mm@kvack.org