VFS: default: add support for FILE_ATTRIBUTE_OFFLINE to async dosmode

VFS: default: add support for FILE_ATTRIBUTE_OFFLINE to async dosmode

This had been missing in the initial async dosmode implementation. It's the
responsibility of the sync and async dosmode functions to call
vfswrap_is_offline() since the offline functionality has been converted from a
first class VFS function to be a part of the DOS attributes VFS functions.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14293

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>