fs: add FMODE_BUF_RASYNC

If set, this indicates that the file system supports IOCB_WAITQ for
buffered reads.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/fs.h

@@ -175,6 +175,9 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 /* File does not contribute to nr_files count */
 #define FMODE_NOACCOUNT		((__force fmode_t)0x20000000)
 
+/* File supports async buffered reads */
+#define FMODE_BUF_RASYNC	((__force fmode_t)0x40000000)
+
 /*
  * Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
  * that indicates that they should check the contents of the iovec are