cifs: Replace wrtPending with a real reference count

cifs: Replace wrtPending with a real reference count

Currently, cifs_close() tries to wait until all I/O is complete and then
frees the file private data.  If I/O does not completely in a reasonable
amount of time it frees the structure anyway, leaving a potential use-
after-free situation.

This patch changes the wrtPending counter to a complete reference count and
lets the last user free the structure.

Signed-off-by: Dave Kleikamp <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Tested-by: Shirish Pargaonkar <[email protected]>
Signed-off-by: Steve French <[email protected]>