fusefs: Fix iosize for FUSE_WRITE in 7.8 compat mode
When communicating with a FUSE server that implements version 7.8
(or older) of the FUSE protocol, the FUSE_WRITE request structure is
16 bytes shorter than normal. The protocol version check wasn't
applied universally, leading to an extra 16 bytes being sent to such
servers. The extra bytes were allocated and bzero()d, so there was
no information disclosure.