mds: CInode::build_backtrace() always incr iter

Always increment the iterator when adding old pools
to the backtrace.  This fixes a bug on files where
the layout had been set to a different pool and then
back to the same pool, causing continuous looping in
the build_backtrace() function.

Fixes #4537.
Signed-off-by: Sam Lang <sam.lang@inktank.com>