imsm: fix spare record writeout race

imsm: fix spare record writeout race

imsm_activate_spare() in the manager thread may race against
write_super_imsm_spares() in the monitor thread.  Give
write_super_imsm_spares() its own private mpb buffer to prevent
confusing the manager.

This change uncovered cases where spares were not being assembled due to
a failed metadata version number check.  Spares can freely associate
across metadata version number, so reduce the scope of the version check
in the spare assembly case.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>