osd/ECExtentCache: convert LRU::Key from class to struct

osd/ECExtentCache: convert LRU::Key from class to struct

Change Key from a class with public members and constructor to a
plain struct. Since Key only holds data with no special initialization
logic, a struct with aggregate initialization is more appropriate
and simpler.

This removes the user-declared constructor and relies on the compiler's
default aggregate initialization from initializer lists.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>