mlx4: use bitmap_find_next_zero_area

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Roland Dreier <rolandd@cisco.com>
Cc: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>