Add port of FreeBSD 'volmode' property

The volmode property may be set to control the visibility of ZVOL
block devices.

This allow switching ZVOL between three modes:
   full - existing fully functional behaviour (default)
   dev  - hide partitions on ZVOL block devices
   none - not exposing volumes outside ZFS

Additionally the new zvol_volmode module parameter can be used to
control the default behaviour.

This functionality can be used, for instance, on "backup" pools to
avoid cluttering /dev with unneeded zd* devices.

Original-patch-by: mav <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>

FreeBSD-commit: https://github.com/freebsd/freebsd/commit/dd28e6bb
Closes #1796 
Closes #3438 
Closes #6233

man/man5/zfs-module-parameters.5

@@ -2076,6 +2076,18 @@ Max number of threads which can handle zvol I/O requests concurrently.
 Default value: \fB32\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzvol_volmode\fR (uint)
+.ad
+.RS 12n
+Defines zvol block devices behaviour when \fBvolmode\fR is set to \fBdefault\fR.
+Valid values are \fB1\fR (full), \fB2\fR (dev) and \fB3\fR (none).
+.sp
+Default value: \fB1\fR.
+.RE
+
 .sp
 .ne 2
 .na

man/man8/zfs.8

@@ -1757,6 +1757,35 @@ when the pool is low on space.
 For a sparse volume, changes to
 .Sy volsize
 are not reflected in the reservation.
+.It Sy volmode Ns = Ns Cm default | full | geom | dev | none
+This property specifies how volumes should be exposed to the OS.
+Setting it to
+.Sy full
+exposes volumes as fully fledged block devices, providing maximal
+functionality. The value
+.Sy geom
+is just an alias for
+.Sy full
+and is kept for compatibility.
+Setting it to
+.Sy dev
+hides its partitions.
+Volumes with property set to
+.Sy none
+are not exposed outside ZFS, but can be snapshoted, cloned, replicated, etc,
+that can be suitable for backup purposes.
+Value
+.Sy default
+means that volumes exposition is controlled by system-wide tunable
+.Va zvol_volmode ,
+where
+.Sy full ,
+.Sy dev
+and
+.Sy none
+are encoded as 1, 2 and 3 respectively.
+The default values is
+.Sy full .
 .It Sy vscan Ns = Ns Sy on Ns | Ns Sy off
 Controls whether regular files should be scanned for viruses when a file is
 opened and closed.