f2fs: check the level before calling get_nid function
f2fs: check the level before calling get_nid function

The caller of get_nid should be careful not to put lower value than
NODE_DIR1_BLOCK in case of level is zero.

Signed-off-by: Changman Lee <f10da886ecf5f6d400f766170992d382feccd416@samsung.com>
Reviewed-by: Namjae Jeon <65d484aa1efdaf0ae258f87a04ed8ec8685cba3e@samsung.com>
Signed-off-by: Jaegeuk Kim <20424effba9d4aeeb410130c5f23aa328fa29e09@samsung.com>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas