kdump: use print_mask_arg0 appropriately

Some callers are using print_mask_arg() when they should be using
print_mask_arg0(); the latter should be used when all flags are optional
and there's not a flag to be decoded with a 0-mask. This turns:

nmount(0x6991e009000,0x8,0<><invalid>0)

into:

nmount(0x6991e009000,0x8,0)

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D43007