HAMMER VFS - Implement inode number matching against the directory hash

HAMMER VFS - Implement inode number matching against the directory hash

Change hammer_create_inode() and hammer_alloc_objid() to take the upper
10 bits of the filename hash and attempt to use them as the lower 10 bits
of the inode number.

hammer_alloc_objid() now utilizes a small two-level 1024-bit bitmap to
assign the low 10 bits of the inode number, on a per-directory basis.
If a collision occurs a nearby value is used instead.

The idea is for inode numbers to have approximately the same linearity
as the directory read scan order, so programs such as cpio, tar, cpdup,
find, and ls which do recursive scans of the directory tree get much
better locality of reference for stat() and other related system calls.

Based-on: Numerous discussions with people, esp corecode.