LU-17336 gss: fix __user pointer in rsi_upcall_seq_write

LU-17336 gss: fix __user pointer in rsi_upcall_seq_write

rsi_upcall_seq_write() uses sscanf to get the string passed from
userspace, but this needs to be copied to a kernel buffer first.

Lustre-change: https://review.whamcloud.com/53342
Lustre-commit: TBD (from 523ffed1cb43eec5fac38c144967026308da9cad)

Test-Parameters: trivial
Test-Parameters: kerberos=true testlist=sanity-krb5
Test-Parameters: testgroup=review-dne-selinux-ssk-part-2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I2ec875b7c6c158695857fe912ec1dd9f41ddc25d
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53434
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>