reiserfs: cleanup, rename key and item accessors to more friendly names

This patch does a quick search and replace:
B_N_PITEM_HEAD() -> item_head()
B_N_PDELIM_KEY() -> internal_key()
B_N_PKEY() -> leaf_key()
B_N_PITEM() -> item_body()

And the item_head version:
B_I_PITEM() -> ih_item_body()
I_ENTRY_COUNT() -> ih_entry_count()

And the treepath variants:
get_ih() -> tp_item_head()
PATH_PITEM_HEAD() -> tp_item_head()
get_item() -> tp_item_body()

... which makes the code much easier on the eyes.

I've also removed a few unused macros.

Checkpatch will complain about the 80 character limit for do_balan.c.
I've addressed that in a later patchset to split up balance_leaf().

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