ext4: reduce computation of overhead during resize

This patch avoids doing an O(n**2)-complexity walk through every flex group.
Instead, it uses the already computed overhead information for the newly
allocated space, and simply adds it to the previously calculated
overhead stored in the superblock.  This drastically reduces the time
taken to resize very large bigalloc filesystems (from 3+ hours for a
64TB fs down to milliseconds).

Signed-off-by: Oleg Kiselev <okiselev@amazon.com>
Link: https://lore.kernel.org/r/CE4F359F-4779-45E6-B6A9-8D67FDFF5AE2@amazon.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>