newbus: Simplify reallocation of devices array

Simplify the complex expression down to what it is on 64-bit systems.
32-bit kernels would allocate 2 at a time. Replace all that with
reallocf which will eliminate a bunch of copies. This should be faster
and simpler on both types of kernels. In addition, transition to
M_WAITOK since this is a sleepable context.

Suggested by: jhb
Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D47362