9p: apply review requests for fid refcounting

Fix style issues in parent commit ("apply review requests for fid
refcounting"), no functional change.

Link: http://lkml.kernel.org/r/1605802012-31133-2-git-send-email-asmadeus@codewreck.org


Fixes: 6636b6dc ("9p: add refcount to p9_fid struct")
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>