cachefiles: fix dentry leak in cachefiles_open_file()

A dentry leak may be caused when a lookup cookie and a cull are concurrent:

P1 | P2
-----------------------------------------------------------
cachefiles_lookup_cookie
cachefiles_look_up_object
lookup_one_positive_unlocked
// get dentry
cachefiles_cull
inode->i_flags |= S_KERNEL_FILE;
cachefiles_open_file
cachefiles_mark_inode_in_use
__cachefiles_mark_inode_in_use
can_use = false
if (!(inode->i_flags & S_KERNEL_FILE))
can_use = true
return false
return false
// Returns an error but doesn't put dentry

After that the following WARNING will be triggered when the backend folder
is umounted:

==================================================================
BUG: Dentry 000000008ad87947{i=7a,n=Dx_1_1.img} still in use (1) [unmount of ext4 sda]
WARNING: CPU: 4 PID: 359261 at fs/dcache.c:1767 umount_check+0x5d/0x70
CPU: 4 PID: 359261 Comm: umount Not tainted 6.6.0-dirty #25
RIP: 0010:umount_check+0x5d/0x70
Call Trace:
<TASK>
d_walk+0xda/0x2b0
do_one_tree+0x20/0x40
shrink_dcache_for_umount+0x2c/0x90
generic_shutdown_super+0x20/0x160
kill_block_super+0x1a/0x40
ext4_kill_sb+0x22/0x40
deactivate_locked_super+0x35/0x80
cleanup_mnt+0x104/0x160
==================================================================

Whether cachefiles_open_file() returns true or false, the reference count
obtained by lookup_positive_unlocked() in cachefiles_look_up_object()
should be released.

Therefore release that reference count in cachefiles_look_up_object() to
fix the above issue and simplify the code.

Fixes: 1f08c925e7a3 ("cachefiles: Implement backing file wrangling")
Cc: [e-mail]
Signed-off-by: Baokun Li <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Acked-by: David Howells <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/cachefiles/namei.c

···
 	 * write and readdir but not lookup or open).
 	 */
 	touch_atime(&file->f_path);
-	dput(dentry);
 	return true;
 
 check_failed:
 	fscache_cookie_lookup_negative(object->cookie);
 	cachefiles_unmark_inode_in_use(object, file);
 	fput(file);
-	dput(dentry);
 	if (ret == -ESTALE)
 		return cachefiles_create_file(object);
 	return false;
···
 	fput(file);
 error:
 	cachefiles_do_unmark_inode_in_use(object, d_inode(dentry));
-	dput(dentry);
 	return false;
 }
 
···
 		goto new_file;
 	}
 
-	if (!cachefiles_open_file(object, dentry))
+	ret = cachefiles_open_file(object, dentry);
+	dput(dentry);
+	if (!ret)
 		return false;
 
 	_leave(" = t [%lu]", file_inode(object->file)->i_ino);