ext4: improving calculation of 'fe_{len|start}' in mb_find_extent()

ext4: improving calculation of 'fe_{len|start}' in mb_find_extent()

After first execution of mb_find_order_for_block():

'fe_start' is the value of 'block' passed in mb_find_extent().

'fe_len' is the difference between the length of order-chunk and
remainder of the block divided by order-chunk.

And 'next' does not require initialization after above modifications.

Signed-off-by: Gou Hao <gouhao@uniontech.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20231113082617.11258-1-gouhao@uniontech.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>