MDS/CDir: return as early as possible from CDir::should_split_fast()

MDS/CDir: return as early as possible from CDir::should_split_fast()

All we want to know is whether we're above the `fast_limit`; if we're
above that, we don't need to know how many exactly.

By returning early instead of iterating over all entries, a lot of CPU
time can be saved.  In a microbenchmark where `fio` was used to create
thousands of files, the CPU usage of `CDir::should_split_fast()` went
from 6% to less than 1% in the `perf report`.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>