os/bluestore/BlueFS: use uint64_t for `len`

os/bluestore/BlueFS: use uint64_t for `len`

change the type of parameter `len` of `BlueFS::_read_random()` from
`size_t` to `uint64_t`.

i think the type of `size_t` comes from
`rocksdb::RandomAccessFile::Read(uint64_t offset, size_t n,
rocksdb::Slice* result, char* scratch)`. and when we implement this
method, we continued using `n`'s type. but, we are using it with
`std::min()`, for instance, where the template parameter type deduction
fails if the lhs and rhs parameters' types are different. so probaly the
better solution is to use `uint64_t` directly to avoid the the cast and
specializing the template.

Signed-off-by: Kefu Chai <kchai@redhat.com>