staging: gasket: use sizeof(*p) for memory allocation

Use sizeof(*p) instead of sizeof(struct P) for memory allocation. This
change complies with the Linux kernel coding style. It improves
readability and decreases the opportunity for bugs if the pointer
variable type is changed. Issue found by checkpatch.

Signed-off-by: Kimberly Brown <kimbrownkd@gmail.com>
Acked-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>