mm/shmem.c: cast the type of unmap_start to u64

In 64bit system. sb->s_maxbytes of shmem filesystem is MAX_LFS_FILESIZE,
which equal LLONG_MAX.

If offset > LLONG_MAX - PAGE_SIZE, offset + len < LLONG_MAX in
shmem_fallocate, which will pass the checking in vfs_fallocate.

	/* Check for wrap through zero too */
	if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
		return -EFBIG;

loff_t unmap_start = round_up(offset, PAGE_SIZE) in shmem_fallocate
causes a overflow.

Syzkaller reports a overflow problem in mm/shmem:

  UBSAN: Undefined behaviour in mm/shmem.c:2014:10
  signed integer overflow: '9223372036854775807 + 1' cannot be represented in type 'long long int'
  CPU: 0 PID:17076 Comm: syz-executor0 Not tainted 4.1.46+ #1
  Hardware name: linux, dummy-virt (DT)
  Call trace:
     dump_backtrace+0x0/0x2c8 arch/arm64/kernel/traps.c:100
     show_stack+0x20/0x30 arch/arm64/kernel/traps.c:238
     __dump_stack lib/dump_stack.c:15 [inline]
     ubsan_epilogue+0x18/0x70 lib/ubsan.c:164
   ...