ext4: remove unnecessary 'offset' assignment

Although it is in the loop, offset is reassigned at the beginning of the
while loop.  And after the loop, the value will not be used

The clang_analyzer complains as follows:

fs/ext4/dir.c:306:3 warning:

Value stored to 'offset' is never read
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Link: https://lore.kernel.org/r/20211208075307.404703-1-luo.penghao@zte.com.cnSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/dir.c

@@ -303,7 +303,6 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
 			goto done;
 		brelse(bh);
 		bh = NULL;
-		offset = 0;
 	}
 done:
 	err = 0;