arm64: do not include ptrace.h from compat.h

including ptrace.h brings a definition of BITS_PER_PAGE into device
drivers and cause a build warning in allmodconfig builds:

drivers/block/drbd/drbd_bitmap.c:482:0: warning: "BITS_PER_PAGE" redefined
 #define BITS_PER_PAGE  (1UL << (PAGE_SHIFT + 3))

This uses a slightly different way to express current_pt_regs()
that avoids the use of the header and gets away with the already
included asm/ptrace.h.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>