linux: module: weld all but spl.ko into zfs.ko

Originally it was thought it would be useful to split up the kmods
by functionality.  This would allow external consumers to only load
what was needed.  However, in practice we've never had a case where
this functionality would be needed, and conversely managing multiple
kmods can be awkward.  Therefore, this change merges all but the
spl.ko kmod in to a single zfs.ko kmod.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13274

module/zstd/README.md

@@ -9,7 +9,7 @@ library, besides upgrading to a newer ZSTD release.
 
 Tree structure:
 
-* `zfs_zstd.c` is the actual `zzstd` kernel module.
+* `zfs_zstd.c` are the actual `zfs` kernel module hooks.
 * `lib/` contains the unmodified version of the `Zstandard` library
 * `zstd-in.c` is our template file for generating the single-file library
 * `include/`: This directory contains supplemental includes for platform
@@ -25,16 +25,7 @@ To update ZSTD the following steps need to be taken:
 `grep include [path to zstd]/contrib/single_file_libs/zstd-in.c  | awk '{ print $2 }'`
 3. Remove debug.c, threading.c, and zstdmt_compress.c.
 4. Update Makefiles with resulting file lists.
-
-~~~
-
-Note: if the zstd library for zfs is updated to a newer version,
-the macro list in include/zstd_compat_wrapper.h usually needs to be updated.
-this can be done with some hand crafting of the output of the following
-script (on the object file generated from the "single-file library" script in zstd's
-contrib/single_file_libs):
-`nm zstd.o | awk '{print "#define "$3 " zfs_" $3}' > macrotable`
-
+5. Follow symbol renaming notes in `include/zstd_compat_wrapper.h`
 
 ## Altering ZSTD and breaking changes