xfs: fix deadlock retry tracepoint arguments

sc->ip is the inode that's being scrubbed, which means that it's not set
for scrub types that don't involve inodes.  If one of those scrubbers
(e.g. inode btrees) returns EDEADLOCK, we'll trip over the null pointer.
Fix that by reporting either the file being examined or the file that
was used to call scrub.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>