xfs: move xfs_buf_iowait out of (__)xfs_buf_submit

xfs: move xfs_buf_iowait out of (__)xfs_buf_submit

There is no good reason to pass a bool argument to wait for a buffer when
the callers that want that can easily just wait themselves.

This means the wait moves out of the extra hold of the buffer, but as the
callers of synchronous buffer I/O need to hold a reference anyway that is
perfectly fine.

Because all async buffer submitters ignore the error return value, and
the synchronous ones catch the error condition through b_error and
xfs_buf_iowait this also means the new xfs_buf_submit doesn't have to
return an error code.

Signed-off-by: Christoph Hellwig <hch@lst.de>