xfs: Set xfs_buf's b_ops member when zeroing bitmap/summary files

In xfs_growfs_rt(), we enlarge bitmap and summary files by allocating
new blocks for both files. For each of the new blocks allocated, we
allocate an xfs_buf, zero the payload, log the contents and commit the
transaction. Hence these buffers will eventually find themselves
appended to list at xfs_ail->ail_buf_list.

Later, xfs_growfs_rt() loops across all of the new blocks belonging to
the bitmap inode to set the bitmap values to 1. In doing so, it
allocates a new transaction and invokes the following sequence of
functions,
  - xfs_rtfree_range()
    - xfs_rtmodify_range()
      - xfs_rtbuf_get()
        We pass '&xfs_rtbuf_ops' as the ops pointer to xfs_trans_read_buf().
        - xfs_trans_read_buf()
	  We find the xfs_buf of interest in per-ag hash table, invoke
	  xfs_buf_reverify() which ends up assigning '&xfs_rtbuf_ops' to
	  xfs_buf->b_ops.

On the other hand, if xfs_growfs_rt_alloc() had allocated a few blocks
for the bitmap inode and returned with an error, all the xfs_bufs
corresponding to the new bitmap blocks that have been allocated would
continue to be on xfs_ail->ail_buf_list list without ever having a
non-NULL value assigned to their b_ops members. An AIL flush operation
would then trigger the following warning message to be printed on the
console,

  XFS (loop0): _xfs_buf_ioapply: no buf ops on daddr 0x58 len 8
  00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  00000040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  00000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  00000060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  00000070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  CPU: 3 PID: 449 Comm: xfsaild/loop0 Not tainted 5.8.0-rc4-chandan-00038-g4d8c2b9de9ab-dirty #37
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
  Call Trace:
   dump_stack+0x57/0x70
   _xfs_buf_ioapply+0x37c/0x3b0
   ? xfs_rw_bdev+0x1e0/0x1e0
   ? xfs_buf_delwri_submit_buffers+0xd4/0x210
   __xfs_buf_submit+0x6d/0x1f0
   xfs_buf_delwri_submit_buffers+0xd4/0x210
   xfsaild+0x2c8/0x9e0
   ? __switch_to_asm+0x42/0x70
   ? xfs_trans_ail_cursor_first+0x80/0x80
   kthread+0xfe/0x140
   ? kthread_park+0x90/0x90
   ret_from_fork+0x22/0x30

This message indicates that the xfs_buf had its b_ops member set to
NULL.

This commit fixes the issue by assigning "&xfs_rtbuf_ops" to b_ops
member of each of the xfs_bufs logged by xfs_growfs_rt_alloc().

Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>