Add support for zfs mount -R <filesystem>

This commit adds support for mounting a dataset along with all of
it's children with '-R' flag for zfs mount. There can be scenarios
where we want to mount all datasets under one hierarchy instead of
mounting all datasets present on system with '-a' flag.

'-R' flag should work on all root and non-root datasets. Usage
information and man page has been updated for zfs mount. A test
for verifying the behavior for '-R' flag is also added.

Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #16015

man/man8/zfs-mount.8

@@ -43,7 +43,7 @@
 .Cm mount
 .Op Fl Oflv
 .Op Fl o Ar options
-.Fl a Ns | Ns Ar filesystem
+.Fl a Ns | Ns Fl R Ar filesystem Ns | Ns Ar filesystem
 .Nm zfs
 .Cm unmount
 .Op Fl fu
@@ -61,7 +61,7 @@ Displays all ZFS file systems currently mounted.
 .Cm mount
 .Op Fl Oflv
 .Op Fl o Ar options
-.Fl a Ns | Ns Ar filesystem
+.Fl a Ns | Ns Fl R Ar filesystem Ns | Ns Ar filesystem
 .Xc
 Mount ZFS filesystem on a path described by its
 .Sy mountpoint
@@ -83,6 +83,8 @@ for more information.
 .It Fl a
 Mount all available ZFS file systems.
 Invoked automatically as part of the boot process if configured.
+.It Fl R
+Mount the specified filesystems along with all their children.
 .It Ar filesystem
 Mount the specified filesystem.
 .It Fl o Ar options