fd.c: flush data problems on osx

Hi

Current implementation of pg_flush_data when called with zero offset and zero nbytes is assumed to flush all file. In
osxit uses mmap with these arguments, but according to man:  

"[EINVAL]           The len argument was negative or zero. Historically, the system call would not return an
                        error if the argument was zero.  See other potential additional restrictions in the COMPAT-
                        IBILITY section below."

so it is generate a lot of warnings:

"WARNING:  could not mmap while flushing dirty data: Invalid argument"

Call to pg_flush_data with zero offset and nbytes happens when replica starts from base backup and fsync=on. TAP test
toreproduce is attached. 

One possible solution for that is just fallback to pg_fdatasync in case when offset = nbytes = 0.

Also there are no default ifdef inside this function, is there any check that will guarantee that pg_flush_data will
notend up with empty body on some platform? 

---
Stas Kelvich
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company