Trust ARC_BUF_SHARED() more

[amotin]

In my understanding ARC_BUF_SHARED() and arc_buf_is_shared() should return identical results, except the second also asserts it deeper. The first is much cheaper though, saving few pointer dereferences. Replace production arc_buf_is_shared() calls with ARC_BUF_SHARED(), and call arc_buf_is_shared() in random assertions, while making it even more strict.

On my tests this in half reduces arc_buf_destroy_impl() time, that noticeably reduces hash_lock congestion under heavy dbuf eviction.

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.

[grwilson]

Could you still use arc_buf_is_shared in assertions. For example it could be used like this:

		if (!ARC_BUF_SHARED(buf)) {
			abd_copy_to_buf(buf->b_data, hdr->b_l1hdr.b_pabd,
			    arc_buf_size(buf));
		} else {
                         ASSERT(arc_buf_is_shared(buf));
                }

This would provide the strict checks that arc_buf_is_shared provides as a defensive mechanism.

[amotin]

Could you still use arc_buf_is_shared in assertions.

@grwilson I did use it in some, but I've changed/added some more.