VFS: add F_FLUSH_FS_CACHE fcntl

This fcntl requests all cached blocks associated with the minor device
number associated with the regular file are invalidated. If the file
is a block special, invalidate the blocks associated with that minor
device instead.

This is to be used for a test that tests unmapped file-mapped memory
ranges whose blocks are not in the cache and therefore must be fetched
from a FS.

Change-Id: Ide914b2e88413aa90bd731ae587ca06fa5f13ebc

sys/sys/fcntl.h

@@ -326,6 +326,7 @@ __END_DECLS
 
 #if defined(__minix)
 #define F_FREESP       100
+#define F_FLUSH_FS_CACHE	101	/* invalidate cache on associated FS */
 #endif /* defined(__minix) */
 
 #endif /* !_SYS_FCNTL_H_ */