Update fb.c

Default page_size is 4096 from firebird 2.1 

If the user supplies a size smaller than 4096, it will be silently converted to 4096. Other numbers not equal to any of the supported sizes will be silently converted to the next lower supported size. 

http://www.firebirdsql.org/refdocs/langrefupd25-ddl-database.html
Merge pull request #27 from mariuz/patch-1

Update fb.c