loader.efi: refresh size in GetMemoryMap retry loop

If ExitBootServices fails due to a changed efi_mapkey then GetMemoryMap
must be called again. In this case it is also possible for the memory
map to grow, so repeat the initial GetMemoryMap call to fetch the new
size.

Also roll bi_add_efi_data_and_exit into bi_load_efi_data as there's no
need for it to be a separate function.

PR: 202455
Reported by: Berislav Purgar <bpurgar@gmail.com>
Tested by: Berislav Purgar <bpurgar@gmail.com>
Reviewed by: kib
MFC after: 1 week
MFC with: r292338
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4621