ceph: try to allocate a smaller extent map for sparse read

[ Upstream commit aaefabc4 ]

In fscrypt case and for a smaller read length we can predict the
max count of the extent map. And for small read length use cases
this could save some memories.

[ idryomov: squash into a single patch to avoid build break, drop
  redundant variable in ceph_alloc_sparse_ext_map() ]

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Stable-dep-of: 18d44c5d ("ceph: allocate sparse_ext map only for sparse reads")
Signed-off-by: Sasha Levin <sashal@kernel.org>