Fixes for refcounting "struct linux_file" in the LinuxKPI.

Fixes for refcounting "struct linux_file" in the LinuxKPI.

- Allow "struct linux_file" to be refcounted when its "_file" member
  is NULL by using its "f_count" field. The reference counts are
  transferred to the file structure when the file descriptor is
  installed.

- Add missing vdrop() calls for error cases during open().

- Set the "_file" member of "struct linux_file" during open. This
allows use of refcounting through get_file() and fput() with LinuxKPI
character devices.

MFC after:		1 week
Sponsored by:		Mellanox Technologies