smb: propagate error code of extract_sharename()

In addition to the EINVAL, there may be an ENOMEM.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 70431bfd ("cifs: Support fscache indexing rewrite")
Signed-off-by: Katya Orlova <e.orlova@ispras.ru>
Signed-off-by: Steve French <stfrench@microsoft.com>