[SRU Xenial/Yakkety] LP#1636847 -- unexpectedly large memory usage of mounted snaps

[Andy Whitcroft]

Some benchmarking seems to indicate that mounting lots of snaps is
consuming a large amount of kernel memory.  This only seems to happen
when we using more threads than cpus.  This has been traced back to the
SQUASHFS decompressor which seems to consume 100's of MBs of memory when
run in this parallel mode.  While we investigate the true root cause
switch our configuration to match those distros which are unaffected.

Following this email are two patches one for xenial and one for yakkety.

Proposing for SRU to xenial and yakkety.

-apw