Provide accessor function to get number of credits available in a handle
and use it from ext4. Later, computation of available credits won't be
so straightforward.
Reviewed-by: NTheodore Ts'o <tytso@mit.edu>
Signed-off-by: NJan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191105164437.32602-11-jack@suse.czSigned-off-by: NTheodore Ts'o <tytso@mit.edu>