Replace ZFS on Linux references with OpenZFS

This change updates the documentation to refer to the project
as OpenZFS instead ZFS on Linux.  Web links have been updated
to refer to https://github.com/openzfs/zfs.  The extraneous
zfsonlinux.org web links in the ZED and SPL sources have been
dropped.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11007

module/zfs/pathname.c

@@ -73,10 +73,6 @@ pn_alloc_sz(struct pathname *pnp, size_t sz)
 {
 	pnp->pn_buf = kmem_alloc(sz, KM_SLEEP);
 	pnp->pn_bufsize = sz;
-#if 0 /* unused in ZoL */
-	pnp->pn_path = pnp->pn_buf;
-	pnp->pn_pathlen = 0;
-#endif
 }
 
 /*
@@ -89,8 +85,4 @@ pn_free(struct pathname *pnp)
 	kmem_free(pnp->pn_buf, pnp->pn_bufsize);
 	pnp->pn_buf = NULL;
 	pnp->pn_bufsize = 0;
-#if 0 /* unused in ZoL */
-	pnp->pn_path = NULL;
-	pnp->pn_pathlen = 0;
-#endif
 }

module/zfs/spa.c

@@ -1260,7 +1260,7 @@ spa_activate(spa_t *spa, spa_mode_t mode)
 	/*
 	 * This taskq is used to perform zvol-minor-related tasks
 	 * asynchronously. This has several advantages, including easy
-	 * resolution of various deadlocks (zfsonlinux bug #3681).
+	 * resolution of various deadlocks.
 	 *
 	 * The taskq must be single threaded to ensure tasks are always
 	 * processed in the order in which they were dispatched.

module/zfs/vdev_label.c

@@ -1434,7 +1434,7 @@ vdev_uberblock_compare(const uberblock_t *ub1, const uberblock_t *ub2)
 
 	/*
 	 * If MMP_VALID(ub) && MMP_SEQ_VALID(ub) then the host has an MMP-aware
-	 * ZFS, e.g. zfsonlinux >= 0.7.
+	 * ZFS, e.g. OpenZFS >= 0.7.
 	 *
 	 * If one ub has MMP and the other does not, they were written by
 	 * different hosts, which matters for MMP.  So we treat no MMP/no SEQ as