When falloc() was broken into separate falloc_noinstall() and finstall(),
a bug was introduced in kern_openat() such that the error from the vnode
open operation was overwritten before it was passed as an argument to
dupfdopen().  This broke operations on /dev/{stdin,stdout,stderr}.  Fix
by preserving the original error number across finstall() so that it is
still available.

Approved by:	re (xxx)

Index: kern/vfs_syscalls.c
===================================================================
--- kern/vfs_syscalls.c	(revision 224841)
+++ kern/vfs_syscalls.c	(working copy)
@@ -1145,10 +1145,11 @@
 		    (error == ENODEV || error == ENXIO) &&
 		    (td->td_dupfd >= 0)) {
 			/* XXX from fdopen */
+			int error_open = error;
 			if ((error = finstall(td, fp, &indx, flags)) != 0)
 				goto bad_unlocked;
 			if ((error = dupfdopen(td, fdp, indx, td->td_dupfd,
-			    flags, error)) == 0)
+			    flags, error_open)) == 0)
 				goto success;
 		}
 		/*