efi: runtime: Fix potential overflow of soft-reserved region size

[ Upstream commit de1034b3 ]

md_size will have been narrowed if we have >= 4GB worth of pages in a
soft-reserved region.

Signed-off-by: Andrew Bresticker <abrestic@rivosinc.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>