mm/vunmap: add cond_resched() in vunmap_pmd_range

Like zap_pte_range add cond_resched so that we can avoid softlockups as
reported below.  On non-preemptible kernel with large I/O map region (like
the one we get when using persistent memory with sector mode), an unmap of
the namespace can report below softlockups.

22724.027334] watchdog: BUG: soft lockup - CPU#49 stuck for 23s! [ndctl:50777]
 NIP [c0000000000dc224] plpar_hcall+0x38/0x58
 LR [c0000000000d8898] pSeries_lpar_hpte_invalidate+0x68/0xb0
 Call Trace:
    flush_hash_page+0x114/0x200
    hpte_need_flush+0x2dc/0x540
    vunmap_page_range+0x538/0x6f0
    free_unmap_vmap_area+0x30/0x70
    remove_vm_area+0xfc/0x140
    __vunmap+0x68/0x270
    __iounmap.part.0+0x34/0x60
    memunmap+0x54/0x70
    release_nodes+0x28c/0x300
    device_release_driver_internal+0x16c/0x280
    unbind_store+0x124/0x170
    drv_attr_store+0x44/0x60
    sysfs_kf_write+0x64/0x90
    kernfs_fop_write+0x1b0/0x290
    __vfs_write+0x3c/0x70
    vfs_write+0xd8/0x260
    ksys_write+0xdc/0x130
    system_call+0x5c/0x70
Reported-by: NHarish Sriram <harish@linux.ibm.com>
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200807075933.310240-1-aneesh.kumar@linux.ibm.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -104,6 +104,8 @@ static void vunmap_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
 		if (pmd_none_or_clear_bad(pmd))
 			continue;
 		vunmap_pte_range(pmd, addr, next, mask);
+
+		cond_resched();
 	} while (pmd++, addr = next, addr != end);
 }