zfs receive and rollback can skew filesystem_count

This commit fixes a small issue which causes both zfs receive and
rollback operations to incorrectly increase the "filesystem_count"
property value.

This change also adds a new test group "limits" to the ZFS Test Suite
to exercise both filesystem_count/limit and snapshot_count/limit
functionality.

Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8232