btrfs-progs: Introduce a misc test for thread conflict in btrfs-convert

btrfs-progs: Introduce a misc test for thread conflict in btrfs-convert

Current code of btrfs-convert have a bug of thread conflict, which caused
invalid memory accessing between threads, and make program panic.

This patch add a test item for above bug, as:
 # ./misc-tests.sh
    [TEST]   001-btrfstune-features
    [TEST]   002-uuid-rewrite
    [TEST]   003-zero-log
    [TEST]   004-convert-thread-conflict
 failed: btrfs-convert /root/btrfsprogs/tests/test.img
 test failed for case 004-convert-thread-conflict
 #
 # cat misc-tests-results.txt
 ...
 ############### btrfs-convert /root/btrfsprogs/tests/test.img
 trans 7 running 5
 ctree.c:363: btrfs_cow_block: Assertion `1` failed.
 btrfs-convert(btrfs_cow_block+0x92)[0x40acaf]
 btrfs-convert(btrfs_search_slot+0x1cb)[0x40c50f]
 btrfs-convert(btrfs_csum_file_block+0x20f)[0x41d83a]
 btrfs-convert[0x43422d]
 btrfs-convert[0x4342cd]
 btrfs-convert[0x4345ca]
 btrfs-convert[0x434767]
 btrfs-convert[0x435770]
 btrfs-convert[0x439748]
 btrfs-convert(main+0x13f8)[0x43b09d]
 /lib64/libc.so.6(__libc_start_main+0xfd)[0x335e01ecdd]
 btrfs-convert[0x407649]
 create btrfs filesystem:
         blocksize: 4096
         nodesize:  16384
         features:  extref, skinny-metadata (default)
 creating btrfs metadata.

 creating ext2fs image file.
 failed: btrfs-convert /root/btrfsprogs/tests/test.img
 test failed for case 004-convert-thread-conflict
 #

Note that this bug is not happened every time, especilly in slow
device as loop(slow cpu with fast block device is likely to trigger).
I set loop count to 20 to make bug happened in 90% tests.

Suggested-by: David Sterba <dsterba@suse.com>
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>