ext4: Reserve revoke credits for freed blocks

So far we have reserved only relatively high fixed amount of revoke
credits for each transaction. We over-reserved by large amount for most
cases but when freeing large directories or files with data journalling,
the fixed amount is not enough. In fact the worst case estimate is
inconveniently large (maximum extent size) for freeing of one extent.

We fix this by doing proper estimate of the amount of blocks that need
to be revoked when removing blocks from the inode due to truncate or
hole punching and otherwise reserve just a small amount of revoke
credits for each transaction to accommodate freeing of xattrs block or
so.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191105164437.32602-23-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>