iov_iter: Introduce fault_in_iov_iter_writeable

iov_iter: Introduce fault_in_iov_iter_writeable

BugLink: https://bugs.launchpad.net/bugs/1976135
commit cdd591fc86e38ad3899196066219fbbd845f3162 upstream

Introduce a new fault_in_iov_iter_writeable helper for safely faulting
in an iterator for writing.  Uses get_user_pages() to fault in the pages
without actually writing to them, which would be destructive.

We'll use fault_in_iov_iter_writeable in gfs2 once we've determined that
the iterator passed to .read_iter isn't in memory.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>