cifs: fix memory leak of smb3_fs_context_dup::server_hostname

cifs: fix memory leak of smb3_fs_context_dup::server_hostname

Fix memory leak of smb3_fs_context_dup::server_hostname when parsing
and duplicating fs contexts during mount(2) as reported by kmemleak:

  unreferenced object 0xffff888125715c90 (size 16):
    comm "mount.cifs", pid 3832, jiffies 4304535868 (age 190.094s)
    hex dump (first 16 bytes):
      7a 65 6c 64 61 2e 74 65 73 74 00 6b 6b 6b 6b a5  zelda.test.kkkk.
    backtrace:
      [<ffffffff8168106e>] kstrdup+0x2e/0x60
      [<ffffffffa027a362>] smb3_fs_context_dup+0x392/0x8d0 [cifs]
      [<ffffffffa0136353>] cifs_smb3_do_mount+0x143/0x1700 [cifs]
      [<ffffffffa02795e8>] smb3_get_tree+0x2e8/0x520 [cifs]
      [<ffffffff817a19aa>] vfs_get_tree+0x8a/0x2d0
      [<ffffffff8181e3e3>] path_mount+0x423/0x1a10
      [<ffffffff8181fbca>] __x64_sys_mount+0x1fa/0x270
      [<ffffffff83ae364b>] do_syscall_64+0x3b/0x90
      [<ffffffff83c0007c>] entry_SYSCALL_64_after_hwframe+0x44/0xae
  unreferenced object 0xffff888111deed20 (size 32):
    comm "mount.cifs", pid 3832, jiffies 4304536044 (age 189.918s)
    hex dump (first 32 bytes):
      44 46 53 52 4f 4f 54 31 2e 5a 45 4c 44 41 2e 54  DFSROOT1.ZELDA.T
      45 53 54 00 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  EST.kkkkkkkkkkk.
    backtrace:
      [<ffffffff8168118d>] kstrndup+0x2d/0x90
      [<ffffffffa027ab2e>] smb3_parse_devname+0x9e/0x360 [cifs]
      [<ffffffffa01870c8>] cifs_setup_volume_info+0xa8/0x470 [cifs]
      [<ffffffffa018c469>] connect_dfs_target+0x309/0xc80 [cifs]
      [<ffffffffa018d6cb>] cifs_mount+0x8eb/0x17f0 [cifs]
      [<ffffffffa0136475>] cifs_smb3_do_mount+0x265/0x1700 [cifs]
      [<ffffffffa02795e8>] smb3_get_tree+0x2e8/0x520 [cifs]
      [<ffffffff817a19aa>] vfs_get_tree+0x8a/0x2d0
      [<ffffffff8181e3e3>] path_mount+0x423/0x1a10
      [<ffffffff8181fbca>] __x64_sys_mount+0x1fa/0x270
      [<ffffffff83ae364b>] do_syscall_64+0x3b/0x90
      [<ffffffff83c0007c>] entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 7be3248f3139 ("cifs: To match file servers, make sure the server hostname matches")
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>