btrfs-progs: tests: remove variable quotation from convert-tests

btrfs-progs: tests: remove variable quotation from convert-tests

In btrfs-progs-v4.11-rc1, the following convert-tests failed.

    [TEST/conv]   008-readonly-image
    [TEST/conv]     readonly image test, btrfs defaults
failed: mke2fs -t ext4 -b 4096 -F /Build/btrfs-progs-v4.11-rc1/tests/test.img
test failed for case 008-readonly-image
Makefile:271: recipe for target 'test-convert' failed
make: *** [test-convert] Error 1
    [TEST/conv]   009-common-inode-flags
    [TEST/conv]     common inode flags test, btrfs defaults
failed: mke2fs -t ext4 -b 4096 -F /Build/btrfs-progs-v4.11-rc1/tests/test.img
test failed for case 009-common-inode-flags
Makefile:271: recipe for target 'test-convert' failed
make: *** [test-convert] Error 1

So, remove quotes from $default_mke2fs.

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