netfs: Fix netfs_page_mkwrite() to check folio->mapping is valid

In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix netfs_page_mkwrite() to check folio->mapping is valid

Fix netfs_page_mkwrite() to check that folio->mapping is valid once it has

taken the folio lock (as filemap_page_mkwrite() does). Without this,

generic/247 occasionally oopses with something like the following:

BUG: kernel NULL pointer dereference, address: 0000000000000000

#PF: supervisor read access in kernel mode

#PF: error_code(0x0000) - not-present page

RIP: 0010:trace_event_raw_event_netfs_folio+0x61/0xc0

...

Call Trace:

<TASK>

? __die_body+0x1a/0x60

? page_fault_oops+0x6e/0xa0

? exc_page_fault+0xc2/0xe0

? asm_exc_page_fault+0x22/0x30

? trace_event_raw_event_netfs_folio+0x61/0xc0

trace_netfs_folio+0x39/0x40

netfs_page_mkwrite+0x14c/0x1d0

do_page_mkwrite+0x50/0x90

do_pte_missing+0x184/0x200

__handle_mm_fault+0x42d/0x500

handle_mm_fault+0x121/0x1f0

do_user_addr_fault+0x23e/0x3c0

exc_page_fault+0xc2/0xe0

asm_exc_page_fault+0x22/0x30

This is due to the invalidate_inode_pages2_range() issued at the end of the

DIO write interfering with the mmap'd writes.