staging/lustre/ptlrpc: Adjust NULL comparison codestyle

All instances of "x == NULL" are changed to "!x" and
"x != NULL" to "x"

Also remove some redundant assertions.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>