libminixfs: add support for peeking blocks

libminixfs: add support for peeking blocks

With this change, the lmfs_get_block*(3) functions allow the caller to
specify that it only wants the block if it is in the cache or the
secondary VM cache.  If the block is not found there, the functions
return NULL.  Previously, the PREFETCH method would be used to this
end instead, which was both abuse in name and less efficient.

Change-Id: Ieb5a15b67fa25d2008a8eeef9d126ac908fc2395