SMB3: Honor 'posix' flag for multiuser mounts

The flag from the primary tcon needs to be copied into the volume info
so that cifs_get_tcon will try to enable extensions on the per-user
tcon. At that point, since posix extensions must have already been
enabled on the superblock, don't try to needlessly adjust the mount
flags.

Fixes: ce558b0e ("smb3: Add posix create context for smb3.11 posix mounts")
Fixes: b326614e ("smb3: allow "posix" mount option to enable new SMB311 protocol extensions")
Signed-off-by: Paul Aurich <paul@darkrain42.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>