staging: gasket: Convert get_user_pages*() --> pin_user_pages*()

staging: gasket: Convert get_user_pages*() --> pin_user_pages*()

In 2019, we introduced pin_user_pages*() and now we are converting
get_user_pages*() to the new API as appropriate. [1] & [2] could
be referred for more information.

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
	https://lwn.net/Articles/807108/

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/1592361930-3813-1-git-send-email-jrdr.linux@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>