efi/libstub: Don't parse overlong command lines

Check if the command line passed in is larger than COMMAND_LINE_SIZE,
and truncate it to the last full argument if so.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200521002921.69650-1-nivedita@alum.mit.edu


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