reiserfs: use buffer_info for leaf_paste_entries

This patch makes leaf_paste_entries more consistent with respect to the
other leaf operations.  Using buffer_info instead of buffer_head
directly allows us to get a superblock pointer for use in error
handling.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>