raid1 uses MD_SB_DISKS to size two data structures,
but the new version-1 superblock allows for more than
this number of disks (and most actual arrays use many
fewer).
This patch sizes to two arrays dynamically.
One becomes a separate kmalloced array.
The other is moved to the end of the containing structure
and appropriate extra space is allocated.

Also, change r1buf_pool_alloc (which allocates buffers for
a mempool for doing re-sync) to not get r1bio structures
from the r1bio pool (which could exhaust the pool) but instead
to allocate them separately.