stat(2): report mount points using st_bsdflags SFBSD_MNTPOINT flag

Reviewed by:	jah, sobomax, markj, olce
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59910

sys/kern/vfs_syscalls.c

@@ -2570,6 +2570,8 @@ kern_statat(struct thread *td, int flag, int fd, const char *path,
 	}
 	error = VOP_STAT(nd.ni_vp, sbp, td->td_ucred, NOCRED);
 	NDFREE_PNBUF(&nd);
+	if (error == 0 && (nd.ni_vp->v_vflag & VV_ROOT) != 0)
+		sbp->st_bsdflags |= SFBSD_MNTPOINT;
 	vput(nd.ni_vp);
 #ifdef __STAT_TIME_T_EXT
 	sbp->st_atim_ext = 0;

sys/kern/vfs_vnops.c

@@ -1838,6 +1838,8 @@ vn_statfile(struct file *fp, struct stat *sb, struct ucred *active_cred)
 
 	vn_lock(vp, LK_SHARED | LK_RETRY);
 	error = VOP_STAT(vp, sb, active_cred, fp->f_cred);
+	if (error == 0 && (vp->v_vflag & VV_ROOT) != 0)
+		sb->st_bsdflags |= SFBSD_MNTPOINT;
 	VOP_UNLOCK(vp);
 
 	return (error);

sys/sys/stat.h

@@ -343,6 +343,7 @@ struct nstat {
 
 /* st_bsdflags */
 #define	SFBSD_NAMEDATTR	0x0001		/* file is named attribute or dir */
+#define	SFBSD_MNTPOINT	0x0002		/* file is a mount point */
 
 #ifdef _KERNEL
 /*