percpu_ref: replace pcpu_ prefix with percpu_

percpu_ref uses pcpu_ prefix for internal stuff and percpu_ for
externally visible ones.  This is the same convention used in the
percpu allocator implementation.  It works fine there but percpu_ref
doesn't have too much internal-only stuff and scattered usages of
pcpu_ prefix are confusing than helpful.

This patch replaces all pcpu_ prefixes with percpu_.  This is pure
rename and there's no functional change.  Note that PCPU_REF_DEAD is
renamed to __PERCPU_REF_DEAD to signify that the flag is internal.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Kent Overstreet <kmo@daterainc.com>