ext4: remove unnecessary 'offset' assignment

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.cn
Signed-off-by: Theodore Ts'o <tytso@mit.edu>