ntfs: take invalidate_lock in ntfs_filemap_page_mkwrite()

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

ntfs: take invalidate_lock in ntfs_filemap_page_mkwrite()

ntfs_filemap_page_mkwrite() calls iomap_page_mkwrite() without holding
mapping->invalidate_lock, so a concurrent truncate or fallocate can be
in the middle of invalidating pagecache and rewriting the runlist while
the write fault maps blocks and dirties the folio. This races with
ntfs_attr_fallocate(), which merges clusters into the in-memory
runlist, drops the runlist lock, and only afterwards zeroes the newly
allocated clusters on disk; and with the punch-hole/insert/collapse
paths that free clusters after truncating the cache.

Per Documentation/filesystems/locking.rst, ->page_mkwrite() must ensure
there are no truncate/invalidate races, "usually mapping->invalidate_lock
is suitable for proper serialization". xfs takes its mmaplock (= the
invalidate_lock rwsem) shared in exactly this path.

Take invalidate_lock shared around iomap_page_mkwrite(). The read-only
fault path is already covered because filemap_fault() itself grabs
invalidate_lock shared on instantiation/read paths; only page_mkwrite
was bypassing it in this driver.