Fix: Use vmalloc_sync_mappings on kernel 5.6 as well

Fix: Use vmalloc_sync_mappings on kernel 5.6 as well

Upstream commit [1], that got rid of vmalloc_sync_all and introduced
vmalloc_sync_mappings, is a v5.6 commit:
$ git tag --contains 763802b53a427ed3cbd419dbba255c414fdd9e7c
v5.6
v5.6-rc7
v5.7-rc1
v5.7-rc2
v5.7-rc3

Extend the LINUX_VERSION_CODE check to v5.6 to fix the following warnings:
...
[  483.242037] LTTng: vmalloc_sync_all symbol lookup failed.
[  483.257056] Page fault handler and NMI tracing might trigger faults.
...

[1] https://github.com/torvalds/linux/commit/763802b53a427ed3cbd419dbba255c414fdd9e7c

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>