nfsd: Fix a memory leak when meeting unsupported state_protect_how4

Remember free allocated client when meeting unsupported state protect how.

Fixes: 50c7b948 ("nfsd: minor consolidation of mach_cred handling code")
Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>