f2fs: support fault injection for f2fs_kmem_cache_alloc()

This patch supports to inject fault into f2fs_kmem_cache_alloc().

Usage:
a) echo 32768 > /sys/fs/f2fs/<dev>/inject_type or
b) mount -o fault_type=32768 <dev> <mountpoint>

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>