btrfs-progs: test: fix how to make test files in fsck-tests 013

btrfs-progs: test: fix how to make test files in fsck-tests 013

In my test environment, following error was occurred because the size
of /lib/modules/`uname -r`/* is larger than 1GB.

# make test-fsck
    [TEST]   fsck-tests.sh
    [TEST/fsck]   013-extent-tree-rebuild
failed: cp -aR /lib/modules/4.9.0-rc5/ /test/btrfs-progs/tests/mnt
test failed for case 013-extent-tree-rebuild
Makefile:272: recipe for target 'test-fsck' failed
make: *** [test-fsck] Error 1
#

In this test case, 'generate_dataset small' is enough for making the
test files, so I will use 'generate_dataset' instead of 'cp'.

For this, move 'generate_dataset()' from 'common.convert' to 'common'.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>