btrfs-progs: convert: Fix a bug in rollback check which overwrite return value

btrfs-progs: convert: Fix a bug in rollback check which overwrite return value

Commit 1170ac307900 ("btrfs-progs: convert: Introduce function to check if
convert image is able to be rolled back") reworked rollback check
condition, by checking 1:1 mapping of each file extent.

The idea itself has nothing wrong, but error handler is not implemented
correctly, which over writes the return value and always try to rollback
the fs even it fails to pass the check.

Fix it by correctly return the error before rollback the fs.

Fixes: 1170ac307900 ("btrfs-progs: convert: Introduce function to check if convert image is able to be rolled back")
Reported-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>