cifs: use get_random_u32 for 32-bit lock random

Using get_random_u32 here is faster, more fitting of the use case, and
just as cryptographically secure. It also has the benefit of providing
better randomness at early boot, which is sometimes when this is used.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Steve French <sfrench@samba.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>