vfio: do not set FMODE_LSEEK flag

This file does not support llseek, so don't set the flag advertising it.

Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>