btrfs: return error pointer from alloc_test_extent_buffer

btrfs: return error pointer from alloc_test_extent_buffer

Callers of alloc_test_extent_buffer have not correctly interpreted the
return value as error pointer, as alloc_test_extent_buffer should behave
as alloc_extent_buffer. The self-tests were unaffected but
btrfs_find_create_tree_block could call both functions and that would
cause problems up in the call chain.

Fixes: faa2dbf004e8 ("Btrfs: add sanity tests for new qgroup accounting code")
CC: [email protected] # 4.4+
Signed-off-by: Dan Carpenter <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>