xfs: fix null pointer dereference in tracepoint

xfs: fix null pointer dereference in tracepoint

If dfp is not NULL we exit early here, when dfp is NULL it's allocated
in xfs_defer_alloc() but not assigned. The tracepoint tries to
dereference members of dfp struct.

Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
Cc: stable@vger.kernel.org # v6.8
Fixes: 3f3cec031099c3 ("xfs: force small EFIs for reaping btree extents")
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>