jbd2: Fine tune estimate of necessary descriptor blocks

Currently we reserve j_max_transaction_buffers / 32 for transaction
descriptor blocks. Now that revoke descriptors are accounted for
separately this estimate is unnecessarily high and we can actually
compute much tighter estimate. In the common case of 32k journal blocks
and 4k blocksize this actually reduces the amount of reserved descriptor
blocks from 256 to ~25 which allows us to fit more real data into a
transaction.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191105164437.32602-25-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>