s3/vfs: merge offline functionality into DOS attributes handling

s3/vfs: merge offline functionality into DOS attributes handling

The offline VFS functions predate the SMB_VFS_{GET|SET}_DOS_ATTRIBUTES()
functions, now that we have these, we can use them for the offline
attribute as well.

The primary reason for this is: performance. Merging both functions has
the benefit that in VFS modules that use same backing store bits for
both offline attribute and DOS attributes (like gpfs), we avoid calling
the backing store twice in dos_mode() and file_set_dosmode().

This commit modifies all existing users of the offline attribute to
adapt to the change, the next commit will then remove the obsolete
offline functions.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>