jbd2: Rename h_buffer_credits to h_total_credits

The credit counter now contains both buffer and revoke descriptor block
credits. Rename to counter to h_total_credits to reflect that. No
functional change.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191105164437.32602-21-jack@suse.cz


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