SUNRPC: Fix failures of checksum Kunit tests

Scott reports that when the new GSS krb5 Kunit tests are built as
a separate module and loaded, the RFC 6803 and RFC 8009 checksum
tests all fail, even though they pass when run under kunit.py.

It appears that passing a buffer backed by static const memory to
gss_krb5_checksum() is a problem. A printk in checksum_case() shows
the correct plaintext, but by the time the buffer has been converted
to a scatterlist and arrives at checksummer(), it contains all
zeroes.

Replacing this buffer with one that is dynamically allocated fixes
the issue.

Reported-by: Scott Mayhew <smayhew@redhat.com>
Fixes: 02142b2c ("SUNRPC: Add checksum KUnit tests for the RFC 6803 encryption types")
Tested-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>