efi/printf: Turn vsprintf into vsnprintf

Implement vsnprintf instead of vsprintf to avoid the possibility of a
buffer overflow.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200518190716.751506-17-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>