smb3: Move the flush out of smb2_copychunk_range() into its callers

Move the flush out of smb2_copychunk_range() into its callers.  This will
allow the pagecache to be invalidated between the flush and the operation
in smb3_collapse_range() and smb3_insert_range().

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>