Don't activate metaslabs with weight 0

Don't activate metaslabs with weight 0

We return ENOSPC in metaslab_activate if the metaslab has weight 0,
to avoid activating a metaslab with no space available.  For sanity
checking, we also assert that there is no free space in the range
tree in that case.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #8968