include/elist: Fix clear()

clear() was calling the non-existant function remove(),
use pop_front() instead.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
mds: Remove queued backtraces on inode remove

If replay removes an inode from the mdcache (or
it gets removed somehow else), we need to remove
the scheduled backtraces from the inode (and decrement
the refcount).

We need to go back and garbage collect backtraces too,
right now they're getting leaked.

Signed-off-by: Sam Lang <sam.lang@inktank.com>