mm/hugetlb: fix folio is still mapped when deleted

Migration may be raced with fallocating hole. remove_inode_single_folio
will unmap the folio if the folio is still mapped. However, it's called
without folio lock. If the folio is migrated and the mapped pte has been
converted to migration entry, folio_mapped() returns false, and won't
unmap it. Due to extra refcount held by remove_inode_single_folio,
migration fails, restores migration entry to normal pte, and the folio is
mapped again. As a result, we triggered BUG in filemap_unaccount_folio.

The log is as follows:
BUG: Bad page cache in process hugetlb pfn:156c00
page: refcount:515 mapcount:0 mapping:0000000099fef6e1 index:0x0 pfn:0x156c00
head: order:9 mapcount:1 entire_mapcount:1 nr_pages_mapped:0 pincount:0
aops:hugetlbfs_aops ino:dcc dentry name(?):"my_hugepage_file"
flags: 0x17ffffc00000c1(locked|waiters|head|node=0|zone=2|lastcpupid=0x1fffff)
page_type: f4(hugetlb)
page dumped because: still mapped when deleted
CPU: 1 UID: 0 PID: 395 Comm: hugetlb Not tainted 6.17.0-rc5-00044-g7aac71907bde-dirty #484 NONE
Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
Call Trace:
<TASK>
dump_stack_lvl+0x4f/0x70
filemap_unaccount_folio+0xc4/0x1c0
__filemap_remove_folio+0x38/0x1c0
filemap_remove_folio+0x41/0xd0
remove_inode_hugepages+0x142/0x250
hugetlbfs_fallocate+0x471/0x5a0
vfs_fallocate+0x149/0x380

Hold folio lock before checking if the folio is mapped to avold race with
migration.

Link: https://lkml.kernel.org/r/20250912074139.3575005-1-tujinjiang@huawei.com
Fixes: 4aae8d1c051e ("mm/hugetlbfs: unmap pages if page fault raced with hole punch")
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>