xfs: bugfixes for 3.7-rc5

- fix for large transactions spanning multiple iclog buffers
- zero the allocation_args structure on the stack before using it
  to determine whether to use a worker for allocation
- move allocation stack switch to xfs_bmapi_allocate in order
  to prevent deadlock on AGF buffers
- growfs no longer reads in garbage for new secondary superblocks
- silence a build warning
- ensure that invalid buffers never get written to disk while on
  free list
- don't vmap inode cluster buffers during free
- fix buffer shutdown reference count mismatch
- fix reading of wrapped log data
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