cachefiles: fix memory leak in cachefiles_add_cache()

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

cachefiles: fix memory leak in cachefiles_add_cache()

The following memory leak was reported after unbinding /dev/cachefiles:

==================================================================

unreferenced object 0xffff9b674176e3c0 (size 192):

comm "cachefilesd2", pid 680, jiffies 4294881224

hex dump (first 32 bytes):

01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

backtrace (crc ea38a44b):

[<ffffffff8eb8a1a5>] kmem_cache_alloc+0x2d5/0x370

[<ffffffff8e917f86>] prepare_creds+0x26/0x2e0

[<ffffffffc002eeef>] cachefiles_determine_cache_security+0x1f/0x120

[<ffffffffc00243ec>] cachefiles_add_cache+0x13c/0x3a0

[<ffffffffc0025216>] cachefiles_daemon_write+0x146/0x1c0

[<ffffffff8ebc4a3b>] vfs_write+0xcb/0x520

[<ffffffff8ebc5069>] ksys_write+0x69/0xf0

[<ffffffff8f6d4662>] do_syscall_64+0x72/0x140

[<ffffffff8f8000aa>] entry_SYSCALL_64_after_hwframe+0x6e/0x76

==================================================================

Put the reference count of cache_cred in cachefiles_daemon_unbind() to

fix the problem. And also put cache_cred in cachefiles_add_cache() error

branch to avoid memory leaks.