mm: /proc/pid/smaps_rollup: fix no vma's null-deref

stable inclusion
from stable-v5.10.152
commit a50ed2d28727ff605d95fb9a53be8ff94e8eaaf4
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I73HJ0

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a50ed2d28727ff605d95fb9a53be8ff94e8eaaf4



--------------------------------

Commit 258f669e ("mm: /proc/pid/smaps_rollup: convert to single value
seq_file") introduced a null-deref if there are no vma's in the task in
show_smaps_rollup.

Fixes: 258f669e ("mm: /proc/pid/smaps_rollup: convert to single value seq_file")
Signed-off-by: Seth Jenkins <sethjenkins@google.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Tested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lipeng Sang <sanglipeng1@jd.com>