smb/client: Prevent error pointer dereference

The cifs_sb_tlink() function can return error pointers, but this code
dereferences it before checking for error pointers.  Re-order the code
to fix that.

Fixes: 0f9b6b045bb2 ("fs/smb/client: implement chmod() for SMB3 POSIX Extensions")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>