f2fs: fix negative value for lseek offset

If application throws negative value of lseek with SEEK_DATA|SEEK_HOLE,
previous f2fs went into BUG_ON in get_dnode_of_data, which was reported
by Tommi Rantala.

He could make a simple code to detect this having:
	lseek(fd, -17595150933902LL, SEEK_DATA);

This patch should resolve that bug.
Reported-by: Tommi Rentala <tt.rantala@gmail.com>
[Jaegeuk Kim: relocate the condition as suggested by Chao]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -353,6 +353,8 @@ static loff_t f2fs_llseek(struct file *file, loff_t offset, int whence)
 						maxbytes, i_size_read(inode));
 	case SEEK_DATA:
 	case SEEK_HOLE:
+		if (offset < 0)
+			return -ENXIO;
 		return f2fs_seek_block(file, offset, whence);
 	}