smb: client: fix parsing of source mount option

Handle trailing and leading separators when parsing UNC and prefix
paths in smb3_parse_devname().  Then, store the sanitised paths in
smb3_fs_context::source.

This fixes the following cases

$ mount //srv/share// /mnt/1 -o ...
$ cat /mnt/1/d0/f0
cat: /mnt/1/d0/f0: Invalid argument

The -EINVAL was returned because the client sent SMB2_CREATE "\\d0\f0"
rather than SMB2_CREATE "\d0\f0".

$ mount //srv//share /mnt/1 -o ...
mount: Invalid argument

The -EINVAL was returned correctly although the client only realised
it after sending a couple of bad requests rather than bailing out
earlier when parsing mount options.

Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>