m68k: mm: Add and use "fault.h"

m68k: mm: Add and use "fault.h"

When building with W=1:

    arch/m68k/mm/fault.c:22:5: warning: no previous prototype for ‘send_fault_sig’ [-Wmissing-prototypes]
       22 | int send_fault_sig(struct pt_regs *regs)
	  |     ^~~~~~~~~~~~~~
    arch/m68k/mm/fault.c:68:5: warning: no previous prototype for ‘do_page_fault’ [-Wmissing-prototypes]
       68 | int do_page_fault(struct pt_regs *regs, unsigned long address,
	  |     ^~~~~~~~~~~~~

Fix this by introducing a new header file "fault.h" for holding the
prototypes of functions implemented in arch/m68k/mm/fault.c.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/ef004b8cfe4aac892aa0fb7714c2ed81a02a9b89.1694613528.git.geert@linux-m68k.org