vfs_streams_xattr: remove all uses of fd, use name based functions

vfs_streams_xattr: remove all uses of fd, use name based functions

We don't really need an fd in this module, all calls to the VFS xattr
API can just use the name based versions.

This paves the way for removing the open of the basefile in
streams_xattr_open() in a later commit.

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>