page-alloc: Rename the first_node local variable

page-alloc: Rename the first_node local variable

first_node is the name of a local variable, and part of the nodemask API.  The
only reason this compiles is because the nodemask API is implemented as a
macro rather than an inline function.

It is confusing to read, and breaks when the nodemask API is cleaned up.
Rename the local variable to just 'first' which is still clear in context.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>