Use the jbd2 accessor function for h_buffer_credits.
Reviewed-by: NTheodore Ts'o <tytso@mit.edu>
Signed-off-by: NJan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191105164437.32602-12-jack@suse.czSigned-off-by: NTheodore Ts'o <tytso@mit.edu>