gve: make nic clock reads thread safe

gve: make nic clock reads thread safe

Add a mutex to protect the shared DMA buffer that receives NIC
timestamp reports. The NIC timestamp will be read from two different
threads: the periodic worker and upcoming `gettimex64`.

Move clock registration to the last step of initialization to ensure
that all data needed by the clock module is initialized before
the clock is exposed to usermode.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Joshua Washington <joshwash@google.com>
Signed-off-by: Ankit Garg <nktgrg@google.com>
Signed-off-by: Jordan Rhee <jordanrhee@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20260514225842.110706-3-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>