sysv: fix handling of delete_entry and set_link failures

similar to minixfs series - make sysv_set_link() report failures,
lift dir_put_page() into the callers of sysv_set_link() and
sysv_delete_entry(), make sysv_rename() handle failures in both.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>