efi_loader: efi_mem_carve_out should return s64

efi_mem_carve_out() is used to remove memory pages from a mapping.
As the number of pages to be removed is a 64bit type the return type
should be 64bit too.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>