Fixup the geliboot sector round code

[allanjude]

Summary

Replace all rounding with the round{up,down}2 macros, a missing set of braces caused the previous code to be incorrect Also, replace alloca() with malloc() because alloca() can return an allocation that is actually invalid, causing boot to fail

[tsoome]

sys/boot/i386/libi386/biosdisk.c

749–750

since bd structure is already having bd_sectorsize member, I would suggest to use bd_sectorsize here and below and avoid hardcoded constant; this will help to get ready for non-512B sector disks.

769

bd_io() is returning -1 in case of error, I would keep the same logic, maybe also set errno to ENOMEM.

[allanjude]

Updated with feedback from tsoome

Updating D6213: Fixup the geliboot sector round code

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

Also, replace alloca() with malloc() because alloca() can return an allocation that is...

[allanjude]

Make it actually compile