fs: vfat: reduce the worst case latencies

When a block partition is mounted with FAT file system
and MS_DIRSYNC option is used, some file system operations
like create, rename shall sleep in caller's context until
all the metadata have been committed to the non-volatile memory.
Since this operation is blocking call for user context,
the WRITE_SYNC option must be used instead of WRITE
(async operation) which incur inherent latencies while
flushing the meta-data corresponding to directory entries

Change-Id: I41c514889873a39d564271db0a421e6c66e5ae33
Signed-off-by: xiaogang <xiaogang@codeaurora.org>