namei: Fix use after free in kern_path_locked

In 0ee50b47 ("namei: change filename_parentat() calling
conventions"), filename_parentat() was made to always call putname() on
the  filename before returning, and kern_path_locked() was migrated to
this calling convention.  However, kern_path_locked() uses the "last"
parameter to lookup and potentially create a new dentry. The last
parameter contains the last component of the path and points within the
filename, which was recently freed at the end of filename_parentat().
Thus, when kern_path_locked() calls __lookup_hash(), it is using the
filename after it has already been freed.

In other words, these calling conventions had been wrong for the
only remaining caller of filename_parentat().  Everything else
is using __filename_parentat(), which does not drop the reference;
so should kern_path_locked().

Switch kern_path_locked() to use of __filename_parentat() and move
getting/dropping struct filename into wrapper.  Remove filename_parentat(),
now that we have no remaining callers.

Fixes: 0ee50b47 ("namei: change filename_parentat() calling conventions")
Link: https://lore.kernel.org/linux-fsdevel/YS9D4AlEsaCxLFV0@infradead.org/
Link: https://lore.kernel.org/linux-fsdevel/YS+csMTV2tTXKg3s@zeniv-ca.linux.org.uk/


Cc: Christoph Hellwig <hch@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Reported-by:  <syzbot+fb0d60a179096e8c2731@syzkaller.appspotmail.com>
Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Co-authored-by: Dmitry Kadashev <dkadashev@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>