Make sure not to do I/O for more than MAXPHYS bytes. Doing so can cause

problems in our providers, such as a KASSERT in md(4). We can initiate
I/O for more than MAXPHYS bytes if we've been given a BIO for MAXPHYS
bytes, the blocks from which we're reading couldn't be compressed and
we had compression in preceeding blocks resulting in misalignment of
the blocks we're trying to read relative to the sector. We're forced to
round up the I/O length to make it an multiple of the sector size.

When we detect the condition, we'll reduce the block count and perform
a "short" read. In g_uzip_done() we need to consider the original I/O
length and stop early if we're about to deflate a block that we didn't
read. By using bio_completed in the cloned BIO and not bio_length to
check for this, we automatically and gracefully handle short reads that
our providers may be doing on top of the short reads we may initiate
ourselves.

Obtained from:	Juniper Networks, Inc.

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