Linux: Implement FS_IOC_GETVERSION

Provide access to file generation number on Linux.

Add test coverage.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #12856

tests/zfs-tests/include/commands.cfg

@@ -201,6 +201,7 @@ export ZFSTEST_FILES='badsend
     file_trunc
     file_write
     get_diff
+    getversion
     largest_file
     libzfs_input_check
     mkbusy

tests/zfs-tests/include/libtest.shlib

@@ -4051,6 +4051,20 @@ function stat_crtime #<path>
 	esac
 }
 
+function stat_generation #<path>
+{
+	typeset path=$1
+
+	case $(uname) in
+	Linux)
+		getversion "${path}"
+		;;
+	*)
+		stat -f %v "${path}"
+		;;
+	esac
+}
+
 # Run a command as if it was being run in a TTY.
 #
 # Usage: