reiserfs: cleanup, make hash detection saner

The hash detection code uses long ugly macros multiple times to get the same
value. This patch cleans it up to be easier to read.

[JK: Fixed up path leak in find_hash_out()]

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>