knfsd: nfsd4: silence a compiler warning in ACL code

Silence a compiler warning in the ACL code, and add a comment making clear the
initialization serves no other purpose.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>