thermal: cpu_cooling: don't call kcalloc() under rcu_read_lock

build_dyn_power_table() allocates the power table while holding
rcu_read_lock.  kcalloc using GFP_KERNEL may sleep, so it can't be
called in an RCU read-side path.

Move the rcu protection to the part of the function that really needs
it: the part that handles the dev_pm_opp pointer received from
dev_pm_opp_find_freq_ceil().  In the unlikely case that there is an OPP
added to the cpu while this function is running, return -EAGAIN.

Fixes: c36cf071 ("thermal: cpu_cooling: implement the power cooling device API")
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>