mm/sharepool: Fix UAF reported by KASAN

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PD4P
CVE: NA

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[ 2058.802818][  T290] BUG: KASAN: use-after-free in get_process_sp_res+0x70/0x134
[ 2058.810194][  T290] Read of size 8 at addr ffff00088dc6ab28 by task test_debug_loop/290
[ 2058.820520][  T290] CPU: 5 PID: 290 Comm: test_debug_loop Tainted: G        W  OE     5.10.0+ #2
[ 2058.829377][  T290] Hardware name: EVB(EP) (DT)
[ 2058.833982][  T290] Call trace:
[ 2058.837217][  T290]  dump_backtrace+0x0/0x30c
[ 2058.841660][  T290]  show_stack+0x20/0x30
[ 2058.845758][  T290]  dump_stack+0x120/0x1b0
[ 2058.850028][  T290]  print_address_description.constprop.0+0x2c/0x1fc
[ 2058.856555][  T290]  __kasan_report+0xfc/0x160
[ 2058.861086][  T290]  kasan_report+0x44/0xb0
[ 2058.865356][  T290]  __asan_load8+0x94/0xd0
[ 2058.869623][  T290]  get_process_sp_res+0x70/0x134
[ 2058.874501][  T290]  proc_usage_show+0x1ac/0x304
[ 2058.879208][  T290]...