Several B-tree optimizations

• Introduce first element offset within a leaf. It allows to reduce

by ~50% average memmove() size when adding/removing elements. If the
added/removed element is in the first half of the leaf, we may shift
elements before it and adjust the bth_first instead of moving more
elements after it.

• Use memcpy() instead of memmove() when we know there is no overlap.
• Switch from uint64_t to uint32_t. It does not limit anything,

but 32-bit arches should appreciate it greatly in hot paths.

• Store leaf capacity in struct btree to avoid 64-bit divisions.
• Adjust zfs_btree_insert_into_leaf() to always result in balanced

leaves after splitting, no matter where the new element was inserted.
Not that we care about it much, but it should also allow B-trees with
as little as two elements per leaf instead of 4 previously.

When scrubbing pool of 12 SSDs, storing 1.5TB of 4KB zvol blocks this
reduces amount of time spent in memmove() inside the scan thread from
13.7% to 5.7% and total scrub time by ~15 seconds out of 9 minutes.
It should also reduce spacemaps load time, but I haven't measured it.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #13582