Inline dbuf_findbp() to reduce stack use

Deep recursive call chains are contributing to segfaults in ztest due
to heavy stack use.  Inlining dbuf_findbp() helps reduce the stack depth
of the dbuf_findbp() -> dbuf_hold_impl() cycle.  However, segfaults are
still occurring in this code path, so further reductions are still needed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/dbuf.c

@@ -1490,7 +1490,8 @@ dbuf_clear(dmu_buf_impl_t *db)
 		dbuf_rele(parent, db);
 }
 
-static int
+__attribute__((always_inline))
+static inline int
 dbuf_findbp(dnode_t *dn, int level, uint64_t blkid, int fail_sparse,
     dmu_buf_impl_t **parentp, blkptr_t **bpp)
 {