netfilter: nft_set_hash: try later when GC hits EAGAIN on iteration

Skip GC run if iterator rewinds to the beginning with EAGAIN, otherwise GC
might collect the same element more than once.

Fixes: f6c383b8

 ("netfilter: nf_tables: adapt set backend to use GC transaction API")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>