reiserfs: factor out buffer_info initialization

This is the first in a series of patches to make balance_leaf() not
quite so insane.

This patch factors out the open coded initializations of buffer_info
structures and defines a few initializers for the 4 cases they're used.
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>