ALSA: memalloc: Don't exceed over the requested size

snd_dma_alloc_pages_fallback() tries to allocate pages again when the
allocation fails with reduced size.  But the first try actually
*increases* the size to power-of-two, which may give back a larger
chunk than the requested size.  This confuses the callers, e.g. sgbuf
assumes that the size is equal or less, and it may result in a bad
loop due to the underflow and eventually lead to Oops.

The code of this function seems incorrectly assuming the usage of
get_order().  We need to decrease at first, then align to
power-of-two.

Reported-and-tested-by: he, bo <[email protected]>
Reported-by: zhang jun <[email protected]>
Cc: <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>

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