Turn default_bs and default_ibs into ZFS_MODULE_PARAMs

The default_bs and default_ibs tunables control the default block size
and indirect block size.

So far, default_bs and default_ibs were tunable only on FreeBSD, e.g.,

    sysctl vfs.zfs.default_ibs

Remove the FreeBSD-specific sysctl code and expose default_bs and
default_ibs as tunables on both Linux and FreeBSD using
ZFS_MODULE_PARAM.

One of the use cases for changing the values of those tunables is to
lower the indirect block size, which may improve performance of large
directories (as discussed during the OpenZFS Leadership Meeting
on 2022-08-16).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Mateusz Piotrowski <mateusz.piotrowski@klarasystems.com>
Sponsored-by: Wasabi Technology, Inc.
Closes #14293
(cherry picked from commit 926715b9fc)

man/man4/zfs.4

@@ -15,7 +15,7 @@
 .\" own identifying information:
 .\" Portions Copyright [yyyy] [name of copyright owner]
 .\"
-.Dd December 21, 2022
+.Dd January 10, 2023
 .Dt ZFS 4
 .Os
 .
@@ -228,6 +228,12 @@ relative to the pool.
 Make some blocks above a certain size be gang blocks.
 This option is used by the test suite to facilitate testing.
 .
+.It Sy zfs_default_bs Ns = Ns Sy 9 Po 512 B Pc Pq int
+Default dnode block size as a power of 2.
+.
+.It Sy zfs_default_ibs Ns = Ns Sy 17 Po 128 KiB Pc Pq int
+Default dnode indirect block size as a power of 2.
+.
 .It Sy zfs_history_output_max Ns = Ns Sy 1048576 Ns B Ns B Po 1MB Pc Pq int
 When attempting to log an output nvlist of an ioctl in the on-disk history,
 the output will not be stored if it is larger than this size (in bytes).

module/zfs/dnode.c

@@ -2595,3 +2595,8 @@ EXPORT_SYMBOL(dnode_free_range);
 EXPORT_SYMBOL(dnode_evict_dbufs);
 EXPORT_SYMBOL(dnode_evict_bonus);
 #endif
+
+ZFS_MODULE_PARAM(zfs, zfs_, default_bs, INT, ZMOD_RW,
+	"Default dnode block shift");
+ZFS_MODULE_PARAM(zfs, zfs_, default_ibs, INT, ZMOD_RW,
+	"Default dnode indirect block shift");