Halve the memory being internally allocated by the blist allocator. In
short, half of the memory that is allocated to implement the radix tree is
wasted because we did not change "u_daddr_t" to be a 64-bit unsigned int
when we changed "daddr_t" to be a 64-bit (signed) int. (See r96849 and
r96851.)

Reviewed by: kib, markj
Tested by: pho
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D11028