FROMLIST: userfaultfd/hugetlbfs: Fix minor fault page leak

When uffd-minor enabled, we need to put the page cache before handling the
userfault in hugetlb_no_page(), otherwise the page refcount got leaked.

This can be reproduced by running userfaultfd selftest with hugetlb_shared
mode, then cat /proc/meminfo.

Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: f2bf15fb0969 ("userfaultfd: add minor fault registration mode")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>

Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Link: https://lore.kernel.org/patchwork/patch/1400632/
Bug: 160737021
Bug: 169683130
Change-Id: Iac0ebd6738af8b6212c5a6303e4ee2f482bb5841

mm/hugetlb.c

@@ -4460,6 +4460,7 @@ retry:
 		/* Check for page in userfault range. */
 		if (userfaultfd_minor(vma)) {
 			unlock_page(page);
+			put_page(page);
 			ret = hugetlb_handle_userfault(vma, mapping, idx,
 						       flags, haddr,
 						       VM_UFFD_MINOR);