mm: do not show fs mm pc for VM_LOCKONFAULT pages

When VM_LOCKONFAULT was added, /proc/PID/smaps wasn't hooked up to it, so
looking at /proc/PID/smaps, it shows '??' instead of something
intelligable.  This can be reached by userspace by simply calling
`mlock2(..., MLOCK_ONFAULT);`.

Fix this by adding "lf" to denote VM_LOCKONFAULT.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: de60f5f ("mm: introduce VM_LOCKONFAULT")
Signed-off-by: Jason A. Donenfeld <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Eric B Munson <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: zx2c4

fs/proc/task_mmu.c

@@ -674,6 +674,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 		[ilog2(VM_RAND_READ)]	= "rr",
 		[ilog2(VM_DONTCOPY)]	= "dc",
 		[ilog2(VM_DONTEXPAND)]	= "de",
+		[ilog2(VM_LOCKONFAULT)]	= "lf",
 		[ilog2(VM_ACCOUNT)]	= "ac",
 		[ilog2(VM_NORESERVE)]	= "nr",
 		[ilog2(VM_HUGETLB)]	= "ht",