Revert r288628 and instead fix a discrepancy between the posix_fadvise(2)

man page and POSIX: posix_fadvise(2) returns an error number on failure.

Reported by:	jilles
MFC after:	1 week

lib/libc/sys/posix_fadvise.2

@@ -28,7 +28,7 @@
 .\"	@(#)madvise.2	8.1 (Berkeley) 6/9/93
 .\" $FreeBSD$
 .\"
-.Dd January 30, 2014
+.Dd October 3, 2015
 .Dt POSIX_FADVISE 2
 .Os
 .Sh NAME
@@ -89,11 +89,13 @@ read or written.
 Future access to this data may require a read operation.
 .El
 .Sh RETURN VALUES
-.Rv -std posix_fadvise
-.Sh ERRORS
-The
+If successful,
 .Fn posix_fadvise
-system call returns zero on success, and an error on failure:
+returns zero.
+It returns an error on failure, without setting
+.Va errno .
+.Sh ERRORS
+Possible failure conditions:
 .Bl -tag -width Er
 .It Bq Er EBADF
 The

sys/kern/vfs_syscalls.c

@@ -4663,6 +4663,7 @@ int
 sys_posix_fadvise(struct thread *td, struct posix_fadvise_args *uap)
 {
 
-	return (kern_posix_fadvise(td, uap->fd, uap->offset, uap->len,
-	    uap->advice));
+	td->td_retval[0] = kern_posix_fadvise(td, uap->fd, uap->offset,
+	    uap->len, uap->advice);
+	return (0);
 }