x86/resctrl: Retrieve parent closeid only for monitor groups [REVISIT]

This fixes the below crash while attempting to allocate tasks
to the resctrl root dir,

| estuary:/sys/fs/resctrl$ echo 987 >tasks
| Unable to handle kernel NULL pointer dereference at virtual address
|  0000000000000018
| Mem abort info:
|   ESR = 0x96000006
|   Exception class = DABT (current EL), IL = 32 bits
|   SET = 0, FnV = 0
|   EA = 0, S1PTW = 0
| Data abort info:
|   ISV = 0, ISS = 0x00000006
|   CM = 0, WnR = 0
| user pgtable: 4k pages, 48-bit VAs, pgdp = (____ptrval____)
| [0000000000000018] pgd=0000003f590b7003, pud=0000003f590b8003,
| pmd=0000000000000000
| Internal error: Oops: 96000006 [#1] PREEMPT SMP
| Modules linked in:
| Process ash (pid: 988, stack limit = 0x(____ptrval____))
| CPU: 5 PID: 988 Comm: ash Not tainted 5.2.0-rc1-g543fa121f0c8-dirty #22
| Hardware name: Huawei TaiShan 2280 V2/BC82AMDA, BIOS TA BIOS 2280-A CS V2.15.01 03/02/2019
| pstate: 60400009 (nZCv daif +PAN -UAO)
| pc : rdtgroup_tasks_write+0x188/0x380
| lr : 0x4

| Call trace:
|  rdtgroup_tasks_write+0x188/0x380
|  rdtgroup_file_write+0x1c/0x30
|  kernfs_fop_write+0x114/0x1d8
|  __vfs_write+0x18/0x38
|  vfs_write+0xa4/0x1b0
|  ksys_write+0x68/0xf8
|  __arm64_sys_write+0x18/0x20
|  el0_svc_common.constprop.0+0x58/0xf0
|  el0_svc_handler+0x28/0x78
|  el0_svc+0x8/0xc
| Code: f9402e82 d00069c0 5281b801 f945b400 (b9401842)
| ---[ end trace 8e5cc59c861cbe74 ]---

Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Signed-off-by: James Morse <james.morse@arm.com>