e2fsprogs: Add memory allocation and zero-out helpers

e2fsprogs: Add memory allocation and zero-out helpers

Add functions ext2fs_get_memzero() which will malloc() the memory
using ext2fs_get_mem(), but it will zero the allocated memory afterwards
with memset().

Add function ext2fs_get_arrayzero() which will use calloc() for
allocating and zero-out the array.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>