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