x86: mm/fault.c declare do_page_fault before they get used

declared do_page_fault() in asm-x86/trap.h for both X86_32 and X86_64

removed do_invalid_op declaration from mm/fault.c as it is already declared in asm-x86/trap.h

Signed-off-by: Jaswinder Singh <jaswinder@infradead.org>