mdssvc: reduce pagesize to 50

mdssvc: reduce pagesize to 50

Lastest macOS queries additional file metadata per search result, which causes
the mashalled paged result set including metadata to exceed the 64 KB result
fragment buffer.

Lacking fragementation support in mdssvc (it's supported by the protocol), for
now just reduce the maximum number of results per search page.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15342

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
(cherry picked from commit 724a0518c901589fe1171d94648391832e056f4d)