osd: fix signatures of get_store_errors() and friends

osd: fix signatures of get_store_errors() and friends

The 'store' parameter was ignored by ScrubStore-related get_*_errors()
functions, and is now removed.

The functions are now marked 'const'. That required tagging the caching
member Store::backend as 'mutable'. While 'mutable' is an 'eyesore',
here is one of the rare cases where its use is justified. Following
https://isocpp.org/wiki/faq/const-correctness:
"When methods change the physical but not logical state, the method should
generally be marked as const since it really is an inspector-method."
(The text then continues and specifically prescribes 'mutable' for these situations.)

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>