xfs: bugfixes for 3.8-rc6

- fix return value when filesystem probe finds no XFS magic, a
  regression introduced in 9802182.

- fix stack switch in __xfs_bmapi_allocate by moving the check for stack
  switch up into xfs_bmapi_write.

- fix oops in _xfs_buf_find by validating that the requested block is
  within the filesystem bounds.

- limit speculative preallocation near ENOSPC.

- fix an unmount hang in xfs_wait_buftarg by freeing the
  xfs_buf_log_item in xfs_buf_item_unlock.

- fix a possible use after free with AIO.

- fix xfs_swap_extents after removal of xfs_flushinval_pages, a
  regression introduced in fb59581404a.
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