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