vn_stat: take into account va_blocksize when setting st_blksize

As currently st_blksize is always PAGE_SIZE, it is playing safe to not
use any smaller value.  For some cases this might not be optimal, but
at least nothing should get broken.

Generally I don't expect this commit to change much for the following
reasons (in case of VREG, VDIR):
- application I/O and physical I/O are sufficiently decoupled by
  filesystem code, buffer cache code, cluster and read-ahead logic
- not all applications use st_blksize as a hint, some use f_iosize, some
  use fixed block sizes

I expect writes to the middle of files on ZFS to benefit the most from
this change.

Silence from:	fs@
MFC after:	2 weeks

sys/kern/vfs_vnops.c

@@ -792,11 +792,10 @@ vn_stat(vp, sb, active_cred, file_cred, td)
 	 *   "a filesystem-specific preferred I/O block size for this 
 	 *    object.  In some filesystem types, this may vary from file
 	 *    to file"
-	 * Default to PAGE_SIZE after much discussion.
-	 * XXX: min(PAGE_SIZE, vp->v_bufobj.bo_bsize) may be more correct.
+	 * Use miminum/default of PAGE_SIZE (e.g. for VCHR).
 	 */
 
-	sb->st_blksize = PAGE_SIZE;
+	sb->st_blksize = max(PAGE_SIZE, vap->va_blocksize);
 	
 	sb->st_flags = vap->va_flags;
 	if (priv_check(td, PRIV_VFS_GENERATION))