FileStore: Race condition during object delete is fixed

There was a race condition (hence OSD crash) between lfn_unlink
and lfn_open. The reason was FDCache lookup was called without
taking index lock from lfn_open. Lookup will increase reference
count and thus Clear will not be able to delete those FDs. FDs
will be leaked. The assert within FDCache clear was hitting
because of this.

Fixes: #9480

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
Merge pull request #2510 from somnathr/wip-obj-delete-fix

FileStore: Race condition during object delete is fixed

Reviewed-by: Samuel Just <sam.just@inktank.com>