vfs_streams_xattr: return a fake fd in streams_xattr_open()

vfs_streams_xattr: return a fake fd in streams_xattr_open()

The final step in changing vfs_streams_xattr to not call open() on the
basefile anymore. Instead, we just return a fake file fd based on
dup'ing a pipe fd. Previous commits ensured all calls to VFS API
functions use pathname based versions to do their work.

This ensures we don't trigger kernel oplock breaks for client "open
stream" requests when needlessly opening the basefile.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12791

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Richard Sharpe <realrichardsharpe@gmail.com>
Reviewed-by: Volker Lendecke <vl@samba.org>