silo: Fix ext4 extent resolution.

Several limit tests were off by one, and most crucially for the final
extent walk we didn't backup the extent pointer by one entry like how
all the extent traversal loops require.

Reported-by: Raúl Porcel <armin76@gentoo.org>
Tested-by: Raúl Porcel <armin76@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>