AArch64: Remove TASK_SIZE from mmap_seized

TASK_SIZE for AArch64 may be any of several options between (1 << 39)
and (1 << 48), inclusive, depending on kernel configuration. Go back
to just checking the most significant bit, as was done before commit
3f12d688 was made to accomodate 32-bit
ARM (before mmap_seized got architecture-specific implementations).

This fixes the following error for AArch64 kernels with
CONFIG_ARM64_64K_PAGES=y.

Error (parasite-syscall.c:1105): Can't allocate memory for parasite blob (pid: 104)

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>