btrfs: source common/module when requiring module reload

btrfs: source common/module when requiring module reload

Tests 163, 219, 225 and 242 require the btrfs module to be reloaded
in the middle of the test. If btrfs is compiled builtin, these tests
should be skipped, since we cannot unload it.

Today, this is what happens:

    QA output created by 242
    +./common/btrfs: line 405: _require_loadable_fs_module: command not found

Sourcing common/module in these tests fixes the issue by skipping the
test:
    btrfs/242       [not run] btrfs: must be a module.
    Ran: btrfs/242
    Not run: btrfs/242

Other tests liker btrfs/124 and btrfs/125 already source the same file
for the same reason, so follow the pattern.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>