Use KF_PATH to verify the size of a struct kinfo_file.

Use KF_PATH to verify the size of a struct kinfo_file.

fbsd_core_vnode_path needs to use the offset of the kf_path member of
struct kinfo_file as the minimum size of a struct kinfo_file object.
However, it was using KVE_PATH instead due to a copy and paste bug.

While here, fix another copy and paste bug in the error message for a
truncated kinfo_file object.

gdb/ChangeLog:

	* fbsd-tdep.c (fbsd_core_vnode_path): Use KF_PATH instead of
	KVE_PATH.