alx: acquire mutex for alx_reinit in alx_change_mtu

alx: acquire mutex for alx_reinit in alx_change_mtu

alx_reinit has a lockdep assertion that the alx->mtx mutex must be held.
alx_reinit is called from two places: alx_reset and alx_change_mtu.
alx_reset does acquire alx->mtx before calling alx_reinit.
alx_change_mtu does not acquire this mutex, nor do its callers or any
path towards alx_change_mtu.
Acquire the mutex in alx_change_mtu.

The issue was introduced when the fine-grained locking was introduced
to the code to replace the RTNL. The same commit also introduced the
lockdep assertion.

Fixes: f4eba221099e ("alx: use fine-grained locking instead of RTNL")
Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Link: https://lore.kernel.org/r/20220310232707.44251-1-dossche.niels@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>