floppy: fix invalid pointer dereference in drive_name

This fixes the invalid pointer dereference in the drive_name function of
the floppy driver.

The native_format field of the struct floppy_drive_params is used as
floppy_type array index in the drive_name function.  Thus, the field
should be checked the same way as the autodetect field.

To trigger the bug, one could use a value out of range and set the drive
parameters with the FDSETDRVPRM ioctl.  Next, FDGETDRVTYP ioctl should
be used to call the drive_name.  A floppy disk is not required to be
inserted.

CAP_SYS_ADMIN is required to call FDSETDRVPRM.

The patch adds the check for a value of the native_format field to be in
the '0 <= x < ARRAY_SIZE(floppy_type)' range of the floppy_type array
indices.

The bug was found by syzkaller.

Signed-off-by: Denis Efremov <efremov@ispras.ru>
Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>