fat: Replace prandom_u32() with get_random_u32()

The prandom_u32() is just wrapper of get_random_u32() and has been
removed by commit de492c83

 ("prandom: remove unused functions").
So use get_random_u32() directly.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>