Fixup the geliboot sector rounding code

Replace all rounding with the round{up,down}2 macros
a missing set of braces caused the previous code to be incorrect

replace alloca() with malloc() because alloca() can return an allocation
that is actually invalid, causing boot to fail

Reviewed by: emaste, ed
Thanks To: peter
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D6213