LU-17650 gss: fix use out of bounds in ptlrpc_gss

LU-17650 gss: fix use out of bounds in ptlrpc_gss

KASAN highlighted that the sockaddr_un struct is not enough
for the kernel primitives we use, so we have to use the
bigger sockaddr_storage for allocation, alas the field
names inside are different so we have to jump through some
hoops to make it actually work.
Also for a 128 byte allocation on stack variable is fine and
cannpot fail, so convert to that

Change-Id: I2292900b54756bf39530c96f7c5c228835562bef
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54452
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>