mtd: mtd_subpagetest: convert to use prandom library

mtd: mtd_subpagetest: convert to use prandom library

This removes home-brewed pseudo-random number generator and use
prandom library.

Signed-off-by: Akinobu Mita <[email protected]>
Cc: Artem Bityutskiy <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: David Laight <[email protected]>
Cc: Eilon Greenstein <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Robert Love <[email protected]>
Cc: Valdis Kletnieks <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>