selftests/bpf: Fix decap_sanity_ns cleanup

selftests/bpf: Fix decap_sanity_ns cleanup

decap_sanity prints the following on the 1st run:

    decap_sanity: sh: 1: Syntax error: Bad fd number

and the following on the 2nd run:

    Cannot create namespace file "/run/netns/decap_sanity_ns": File exists

The problem is that the cleanup command has a typo and does nothing.
Fix the typo.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-9-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>