fs/pstore/ram_core.c: replace count*size kmalloc by kmalloc_array

kmalloc_array manages count*sizeof overflow.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>