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.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 327060bd7799e5d4376693fba770a17b79e5a38d)