ceph: fix memory leak in ceph_mds_auth_match()

We now free the temporary target path substring allocation on every
possible branch, instead of omitting the default branch. In some
cases, a memory leak occured, which could rapidly crash the system
(depending on how many file accesses were attempted).

This was detected in production because it caused a continuous memory
growth, eventually triggering kernel OOM and completely hard-locking
the kernel.

Relevant kmemleak stacktrace:

unreferenced object 0xffff888131e69900 (size 128):
comm "git", pid 66104, jiffies 4295435999
hex dump (first 32 bytes):
76 6f 6c 75 6d 65 73 2f 63 6f 6e 74 61 69 6e 65 volumes/containe
72 73 2f 67 69 74 65 61 2f 67 69 74 65 61 2f 67 rs/gitea/gitea/g
backtrace (crc 2f3bb450):
[<ffffffffaa68fb49>] __kmalloc_noprof+0x359/0x510
[<ffffffffc32bf1df>] ceph_mds_check_access+0x5bf/0x14e0 [ceph]
[<ffffffffc3235722>] ceph_open+0x312/0xd80 [ceph]
[<ffffffffaa7dd786>] do_dentry_open+0x456/0x1120
[<ffffffffaa7e3729>] vfs_open+0x79/0x360
[<ffffffffaa832875>] path_openat+0x1de5/0x4390
[<ffffffffaa834fcc>] do_filp_open+0x19c/0x3c0
[<ffffffffaa7e44a1>] do_sys_openat2+0x141/0x180
[<ffffffffaa7e4945>] __x64_sys_open+0xe5/0x1a0
[<ffffffffac2cc2f7>] do_syscall_64+0xb7/0x210
[<ffffffffac400130>] entry_SYSCALL_64_after_hwframe+0x77/0x7f

It can be triggered by mouting a subdirectory of a CephFS filesystem,
and then trying to access files on this subdirectory with an auth token
using a path-scoped capability:

$ ceph auth get client.services
[client.services]
key = REDACTED
caps mds = "allow rw fsname=cephfs path=/volumes/"
caps mon = "allow r fsname=cephfs"
caps osd = "allow rw tag cephfs data=cephfs"

$ cat /proc/self/mounts
services@[REDACTED].cephfs=/volumes/containers /ceph/containers ceph rw,noatime,name=services,secret=<hidden>,ms_mode=prefer-crc,mount_timeout=300,acl,mon_addr=[REDACTED]:3300,recover_session=clean 0 0

$ seq 1 1000000 | xargs -P32 --replace={} touch /ceph/containers/file-{} && \
seq 1 1000000 | xargs -P32 --replace={} cat /ceph/containers/file-{}

[ idryomov: combine if statements, rename rc to path_matched and make
it a bool, formatting ]

Cc: stable@vger.kernel.org
Fixes: 596afb0b8933 ("ceph: add ceph_mds_check_access() helper")
Signed-off-by: Antoine Viallon <antoine@lesviallon.fr>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>