crimson/osd: pass the the cause by string_view

crimson/osd: pass the the cause by string_view

in future, string_view can be used almost every where string can be
used. in this change, `const char*` is instead passed to the constructor
of system_error, as we can ensure that the string_view instances are
always constructed from a `const char*` ended with `\0`.

we need this change for two reasons:

* better performance
* prefer for the world where string_view rules.

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