os/bluestore: simplify spare read path, remove pextent constraint

os/bluestore: simplify spare read path, remove pextent constraint

The previous read code had the constraint that a physical extent had to
be a multiple of the csum chunk size.  This isn't needed: we might have
a csum_block of 1MB and min_alloc_size of 4KB and that's okay.

Collapse the two helpers into a single loop that uses the blob_t::map()
method to do the pextent part of the read.  This is simpler and avoids
the temporary extents2read structure.

Signed-off-by: Sage Weil <sage@redhat.com>