fd: Initialize more export_fd_buf fields in kern_proc_cwd_out()

In particular, we need to initialize efbuf->flags, since
export_vnode_to_sb() loads that field. This was mostly harmless since
the flag only determines whether the output kinfo_file is packed, and
KERN_PROC_CWD only ever emits a single kinfo_file anyway.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation