libxfs: implement get_random_u32

libxfs: implement get_random_u32

Actually query the kernel for some random bytes instead of returning
zero, if that's possible.  The most noticeable effect of this is that
mkfs will now create the rtbitmap file, the rtsummary file, and children
of the root directory with a nonzero generation.  Apparently xfsdump
requires that the root directory have a generation number of zero.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>