libzfs: zfs_mount_at(): load key for encryption root if MS_CRYPT

zfs_crypto_load_key() only works on encryption roots,
and zfs mount -la would fail if it encounters a datasets that
is sorted before their encroots.

To trigger:
  truncate -s 40G /tmp/test
  dd if=/dev/urandom of=/tmp/k bs=128 count=1 status=none
  zpool create -O encryption=on -O keylocation=file:///tmp/k \
               -O keyformat=passphrase test /tmp/test
  zfs create -o mountpoint=/a test/a
  zfs create -o mountpoint=/b test/b
  zfs umount test
  zfs unload-key test
  zfs mount -la

The final mount errored out with:
  Key load error: Keys must be loaded for
    encryption root of 'test/a' (test).
  Key load error: Keys must be loaded for
    encryption root of 'test/b' (test).

And only /test was mounted

This technically breaks the libzfs API, but the previous behavior was
decidedly a bug.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes openzfs#11870 
Closes openzfs#11875

lib/libzfs/libzfs_mount.c

@@ -385,6 +385,9 @@ zfs_mount_at(zfs_handle_t *zhp, const char *options, int flags,
 	struct stat buf;
 	char mntopts[MNT_LINE_MAX];
 	char overlay[ZFS_MAXPROPLEN];
+	char prop_encroot[MAXNAMELEN];
+	boolean_t is_encroot;
+	zfs_handle_t *encroot_hp = zhp;
 	libzfs_handle_t *hdl = zhp->zfs_hdl;
 	uint64_t keystatus;
 	int remount = 0, rc;
@@ -443,7 +446,27 @@ zfs_mount_at(zfs_handle_t *zhp, const char *options, int flags,
 		 */
 		if (keystatus == ZFS_KEYSTATUS_UNAVAILABLE) {
 			if (flags & MS_CRYPT) {
-				rc = zfs_crypto_load_key(zhp, B_FALSE, NULL);
+				rc = zfs_crypto_get_encryption_root(zhp,
+				    &is_encroot, prop_encroot);
+				if (rc) {
+					zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
+					    "Failed to get encryption root for "
+					    "'%s'."), zfs_get_name(zhp));
+					return (rc);
+				}
+
+				if (!is_encroot) {
+					encroot_hp = zfs_open(hdl, prop_encroot,
+					    ZFS_TYPE_DATASET);
+					if (encroot_hp == NULL)
+						return (hdl->libzfs_error);
+				}
+
+				rc = zfs_crypto_load_key(encroot_hp,
+				    B_FALSE, NULL);
+
+				if (!is_encroot)
+					zfs_close(encroot_hp);
 				if (rc)
 					return (rc);
 			} else {