btree cleanups V2

Here's a set of a few small btree cleanup patches. Most of them are in preparation of adding CRC support to the btree blocks. All these require the generic btree patchset to be applied first.

Compared to version 1 I've addressed Dave's review comments. I decided to keep XFS_BMAP_BROOT_PTR_ADDR because it's used a few times and already in long enough lines that would start to get unreadable if we expanded it. I've also left the odd boolean arguments to the maxrecs functions as is for now, but I have some plans to tackle that from a higher level view later.