Fix match_prepath()

Fix match_prepath()

commit cd8c42968ee651b69e00f8661caff32b0086e82d upstream.

Incorrect return value for shares not using the prefix path means that
we will never match superblocks for these shares.

Fixes: commit c1d8b24d1819 ("Compare prepaths when comparing superblocks")
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Cc: Aurélien Aptel <aaptel@suse.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>