ext2: Adjust the comment of function ext2_alloc_branch

The name of argument is error in the header comment.
@num should be @indirect_blks.
At the same time, there was a lack of description of the two parameters
@blks and @goal.
This commit therefore fixes this header comment.

Signed-off-by: Shuning Zhang <sunny.s.zhang@oracle.com>
Signed-off-by: Jan Kara <jack@suse.cz>