ktls: Post receive errors on partially closed sockets.

If an error such as an invalid record or one whose decryption fails is
detected on a socket that has received a RST then ktls_drop() could
ignore the error since INP_DROPPED could already be set. In this case
soreceive_generic hangs since it does not return from a KTLS socket
with pending encrypted data unless there is an error (so_error) (this
behavior is to ensure that soreceive_generic doesn't return a
premature EOF when there is pending data still being decrypted).

Note that this was a bug prior to
69542f26820b7edb8351398b36edda5299c1db56 as tcp_usr_abort would also
have ignored the error in this case.

Reviewed by: gallatin
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D37775

(cherry picked from commit 07be7517270e7cc8f51612efce9e6375ad1fc12e)