ext4: ensure f_bfree returned by ext4_statfs() is non-negative

ext4: ensure f_bfree returned by ext4_statfs() is non-negative

I found the issue that the number of free blocks went negative.
# stat -f /mnt/mp1/
  File: "/mnt/mp1/"
    ID: e175ccb83a872efe Namelen: 255     Type: ext2/ext3
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 258022     Free: -15        Available: -13122
Inodes: Total: 65536      Free: 63029

f_bfree in struct statfs will go negative when the filesystem has
few free blocks. Because the number of dirty blocks is bigger than
the number of free blocks in the following two cases.

CASE 1:
ext4_da_writepages
  mpage_da_map_and_submit
    ext4_map_blocks
      ext4_ext_map_blocks
        ext4_mb_new_blocks
          ext4_mb_diskspace_used
            percpu_counter_sub(&sbi->s_freeblocks_counter, ac->ac_b_ex.fe_len);
        <--- interrupt statfs systemcall --->
        ext4_da_update_reserve_space
            percpu_counter_sub(&sbi->s_dirtyblocks_counter,
                            used + ei->i_allocated_meta_blocks);

CASE 2:
ext4_write_begin
  __block_write_begin
    ext4_map_blocks
      ext4_ext_map_blocks
        ext4_mb_new_blocks
          ext4_mb_diskspace_used
            percpu_counter_sub(&sbi->s_freeblocks_counter, ac->ac_b_ex.fe_len);
            <--- interrupt statfs systemcall --->
            percpu_counter_sub(&sbi->s_dirtyblocks_counter, reserv_blks);

To avoid the issue, this patch ensures that f_bfree is non-negative.

Signed-off-by: Kazuya Mio <k-mio@sx.jp.nec.com>