Remove IO_SYNC flag when writing extended file attributes on ZFS.

While it is possible to create and write file, modify its permissions, etc.
without ever doing sync, it looks odd that it is required for setting
extended file attributes on ZFS.  UFS does not do sync there too.

Samba uses those extended attributes to store some its data, and doing it
synchronously by many times reduces file creation performance for systems
without SLOG device.

Reviewed by:	delphij, jpaetzel, silence on fs@
MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.