The EFI boot loader allocates a single chunk of contiguous memory to
hold the kernel, modules, and any other loaded data. This memory block
is relocated to the kernel's expected location during the transfer of
control from the loader to the kernel.

The GENERIC kernel on amd64 has recently grown such that a kernel + zfs.ko
no longer fits in the default staging size. Bump the default size from
32MB to 48MB to provide more breathing room.

PR: 201679
Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D3666