Just use plain copy_from_user() and get_user().  Note that while a
buf-derived pointer gets stored into ->dxferp, all places that actually use
the resulting value feed it either to import_iovec() or to
import_single_range(), and both will do validation.

Link: https://lore.kernel.org/r/20191017193925.25539-7-viro@ZenIV.linux.org.ukSigned-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>