os/bluestore: fixing a dangling pointer

os/bluestore: fixing a dangling pointer

... and silencing compiler warnings:

Building CXX object src/os/CMakeFiles/os.dir/bluestore/BlueStore.cc.o
../src/os/bluestore/BlueStore.cc:8603:25: warning: object backing the pointer will be destroyed at the end of the full-expression [-Wdangling-gsl]
        const char *c = it->key().c_str();
                        ^~~~~~~~~
../src/os/bluestore/BlueStore.cc:8623:25: warning: object backing the pointer will be destroyed at the end of the full-expression [-Wdangling-gsl]
        const char* c = it->key().c_str();
                        ^~~~~~~~~
(reverting a minor part of ifed01)

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