NFSD: restore trace event lost in mismerge

31ef83dc "nfsd: add trace events" had a typo that dropped a trace
event and replaced it by an incorrect recursive call to
nfsd4_cb_layout_fail.  133d5582

 "Subject: nfsd: don't recursively
call nfsd4_cb_layout_fail" fixed the crash, this restores the
tracepoint.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>