vinyl: fix passing uninitialized parameter to vy_page_find_key()

vy_page_find_key() assumes that equal_key parameter is initialized since
it is used unconditionally. Originally, function was designed with
assumption that parameter is initialized by caller. Since then it has
been used in several other places, but some callers doesn't initialize
this parameter to 'false' value. Let's fix it and inside
vy_page_find_key() set this output parameter to false by default.

Closes #5078