ubifs: Force prandom result to __le32

In set_dent_cookie() the result of prandom_u32() is
assinged to an __le32 type. Make this a forced conversion
to remove the following sparse warning:

fs/ubifs/journal.c:506:30: warning: incorrect type in assignment (different base types)
fs/ubifs/journal.c:506:30:    expected restricted __le32 [usertype] cookie
fs/ubifs/journal.c:506:30:    got unsigned int

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>