Merge the Pull Requests #5824 to branch 0.6.5.11

[wuyan6293]

Merge the Pull Requests #5824 (Reinstate zvol_taskq to fix aio on zvol) from branch 0.7.* to 0.6.5.11.

Motivation and Context

Obviously PR #5824 fixed the AIO on zvol. We want to merge the PR to 0.6.5, thus we can get better performance on zvol with the randread IO mode.
reference:
#5824
692e55b?diff=unified

Description

1. Reinstate the zvol_taskq to support concurrent IO operations
2. We delete the codes about zv_suspend_lock, because this code is added on the commit 040dab9

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)