ext4_ext_index_trans_blocks
extern int ext4_ext_index_trans_blocks(struct inode *inode, int extents);
credits = (2 * ext4_ext_index_trans_blocks(inode, 2)) + 1;
rsv_blocks = 1 + ext4_ext_index_trans_blocks(inode, bpf);
return ext4_ext_index_trans_blocks(inode, pextents);