[PPP]: Fix output buffer size in ppp_decompress_frame().

This patch addresses the issue with "osize too small" errors in mppe
encryption.  The patch fixes the issue with wrong output buffer size
being passed to ppp decompression routine.

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As pointed out by Suresh Mahalingam, the issue addressed by
ppp-fix-osize-too-small-errors-when-decoding patch is not fully resolved yet.
The size of allocated output buffer is correct, however it size passed to
ppp->rcomp->decompress in ppp_generic.c if wrong. The patch fixes that.
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Signed-off-by: Konstantin Sharlaimov <konstantin.sharlaimov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>