buffer: add BH_Prio and BH_Meta flags

Add buffer_head flags so that buffer cache writebacks can be marked
with the the appropriate request flags, so that metadata blocks can be
marked appropriately in blktrace.

Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/buffer.c

···
 	/* Take care of bh's that straddle the end of the device */
 	guard_bh_eod(rw, bio, bh);
 
+	if (buffer_meta(bh))
+		rw |= REQ_META;
+	if (buffer_prio(bh))
+		rw |= REQ_PRIO;
+
 	bio_get(bio);
 	submit_bio(rw, bio);
 

include/linux/buffer_head.h

···
 	BH_Write_EIO,	/* I/O error on write */
 	BH_Unwritten,	/* Buffer is allocated on disk but not written */
 	BH_Quiet,	/* Buffer Error Prinks to be quiet */
+	BH_Meta,	/* Buffer contains metadata */
+	BH_Prio,	/* Buffer should be submitted with REQ_PRIO */
 
 	BH_PrivateStart,/* not a state bit, but the first bit available
 			 * for private allocation by other entities
···
 BUFFER_FNS(Boundary, boundary)
 BUFFER_FNS(Write_EIO, write_io_error)
 BUFFER_FNS(Unwritten, unwritten)
+BUFFER_FNS(Meta, meta)
+BUFFER_FNS(Prio, prio)
 
 #define bh_offset(bh)		((unsigned long)(bh)->b_data & ~PAGE_MASK)