mbcache: use consistent type for entry count

mbcache used several different types to represent the number of entries
in the cache.  For consistency within mbcache and with the shrinker API,
always use unsigned long.

This does not change behavior for current mbcache users (ext2 and ext4)
since they limit the entry count to a value which easily fits in an int.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>