Revert "Btrfs: do not do filemap_write_and_wait_range in fsync"

This reverts commit 0885ef5b



After applying the above patch, the performance slowed down because the dirty
page flush can only be done by one task, so revert it.

The following is the test result of sysbench:
	Before		After
	24MB/s		39MB/s

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>