bc1119d broke things (we weren't checking __wt_lsm_bloom_work for
an error return, but a simple check for an error isn't quite right,
__wt_lsm_bloom_work returns WT_NOTFOUND if no work is done).  The
only caller of __wt_lsm_bloom_work doesn't care about WT_NOTFOUND,
so remove that semantic, and return 0 if no work is done.  This is
not a complete fix, there's still the EBUSY in #1185.