f2fs: fix to adjust appropirate defragment pg_end

A length that exceeds the real size of the inode may be
specified from user, although these out-of-range areas
are not mapped, but they still need to be check in
while loop, which is unnecessary.

Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>