crimson/seastore: fix cache::get_extent got retired extent

crimson/seastore: fix cache::get_extent got retired extent

one transaction got an extent whose state is MUTATION_PENDINGat that time.
but another transaction do split and set the extent state to RETIRED.
when the first transaction resume and do continuation, the state of the extent
has been changed to RETIRED. So need eagain to try again.

Signed-off-by: chunmei-liu <chunmei.liu@intel.com>