ext4: remove redundant variable err

Return value directly from ext4_group_extend_no_check()
instead of getting value from redundant variable err.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jinpeng Cui <cui.jinpeng2@zte.com.cn>
Link: https://lore.kernel.org/r/20220831160843.305836-1-cui.jinpeng2@zte.com.cn


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