mtd: properly check all write ioctls for permissions

commit f7e6b19bc76471ba03725fe58e0c218a3d6266c3 upstream.

When doing a "write" ioctl call, properly check that we have permissions
to do so before copying anything from userspace or anything else so we
can "fail fast".  This includes also covering the MEMWRITE ioctl which
previously missed checking for this.

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[rw: Fixed locking issue]
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>