Decouple Linux files from the belonging character device right after open
in the LinuxKPI. This is done by calling finit() just before returning a magic
value of ENXIO in the "linux_dev_fdopen" function.

The Linux file structure should mimic the BSD file structure as much as
possible. This patch decouples the Linux file structure from the belonging
character device right after the "linux_dev_fdopen" function has returned.
This fixes an issue which allows a Linux file handle to exist after a
character device has been destroyed and removed from the directory index
of /dev. Only when the reference count of the BSD file handle reaches zero,
the Linux file handle is destroyed. This fixes use-after-free issues related
to accessing the Linux file structure after the character device has been
destroyed.

While at it add a missing NULL check for non-present file operation.
Calling a NULL pointer will result in a segmentation fault.

Reviewed by: kib @
MFC after: 1 week
Sponsored by: Mellanox Technologies