zvol_os: fix compile with blk-mq on Linux 4.x

[robn]

Motivation and Context

Since #15992 was merged, master fails to compile on Linux 4.x.

Description

99741bd accesses a cached blk-mq hardware context through the mq_hctx field of struct request. However, this field did not exist until 5.0. Before that, the private function blk_mq_map_queue() was used to dig it out of broader queue context. This commit detects this situation, and handles it with a poor-man's simulation of that function.

How Has This Been Tested?

Compile checked on kernels:

• 4.4.302
• 4.9.337
• 4.14.336
• 5.10.214
• 6.1.38
• 6.2.16
• 6.4.15
• 6.6.23
• 6.8.2
• 6.9.0-rc2

I have not checked if this actually makes the zvol multi taskq works properly on 4.x, or even it works at all.

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)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[robn]

@ixhamza fyi.

[ixhamza]

Thanks @robn, and apologies for missing compiling my changes against old kernels.

[tonyhutter]

Looks like there's a build failure on CentOS 7:
https://build.openzfs.org/builders/CentOS%207%20x86_64%20%28TEST%29/builds/2528/steps/shell_1/logs/make

[ixhamza]

@tonyhutter - I believe that is because #16062 is yet to merge to master.

[ixhamza]

I have not checked if this actually makes the zvol multi taskq works properly on 4.x, or even it works at all.

@robn - JFYI, I have tested on 4.15 with blk-mq enabled to validate that zvol multi taskq functionality works properly with this patch.