search_expr: make nnodes a non-static variable

Static variable nnodes increments whenever a search expression
is allocated. Its purpose is to keep track of the number of
search expressions that are created during search expression tree
normalisation. While uid-based search resets the counter each time
it is called, it may accumulate in process memory as long as only
guidsearch is executed.

This could introduce hard-to-track bugs, as the JMAP email search
code uses the search expression normalisation code independently
from uid-based search.

To avoid this, let's make nnodes a function argument.