f2fs: use is_valid_blkaddr to verify blkaddr for readability

Export is_valid_blkaddr() and use it to replace some codes for readability.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --cc fs/f2fs/checkpoint.c
Simple merge

diff --cc fs/f2fs/f2fs.h
Simple merge

diff --cc fs/f2fs/recovery.c
Simple merge