Use full initializer for initrd_ctx to avoid fatal warnings with older GCC

Use full initializer for initrd_ctx to avoid fatal warnings with older GCC

struct ... foo = { 0, } is valid initializer, but older GCC emits
warning which is fatal error due to -Werror=missing-field-initializer.
So simply use full initializer to avoid these errors. This was fixed
probably in GCC 4.7.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36750