[PATCH] don't apply file size rlimits to blockdevs

generic_file_write()'s rlimit checks are preventing writes to large
offsets into blockdevs:

# ulimit -f 10000
# dd if=/dev/zero of=/dev/sde5 bs=1k count=1 seek=1000000
zsh: file size limit exceeded

So don't apply that check if it's a blockdev.

The patch also caches the S_ISBLK result in a local.

mm/filemap.c

@@ -1530,6 +1530,7 @@ generic_file_write_nolock(struct file *file, const struct iovec *iov,
 	size_t count;		/* after file limit checks */
 	struct inode 	*inode = mapping->host;
 	unsigned long	limit = current->rlim[RLIMIT_FSIZE].rlim_cur;
+	const int	isblk = S_ISBLK(inode->i_mode);
 	long		status = 0;
 	loff_t		pos;
 	struct page	*page;
@@ -1581,14 +1582,12 @@ generic_file_write_nolock(struct file *file, const struct iovec *iov,
 
 	written = 0;
 
+	if (!isblk) {
 		/* FIXME: this is for backwards compatibility with 2.4 */
-	if (!S_ISBLK(inode->i_mode) && file->f_flags & O_APPEND)
+		if (file->f_flags & O_APPEND)
 			pos = inode->i_size;
 
-	/*
-	 * Check whether we've reached the file size limit.
-	 */
-	if (unlikely(limit != RLIM_INFINITY)) {
+		if (limit != RLIM_INFINITY) {
 			if (pos >= limit) {
 				send_sig(SIGXFSZ, current, 0);
 				err = -EFBIG;
@@ -1599,6 +1598,7 @@ generic_file_write_nolock(struct file *file, const struct iovec *iov,
 				count = limit - (u32)pos;
 			}
 		}
+	}
 
 	/*
 	 * LFS rule
@@ -1623,7 +1623,7 @@ generic_file_write_nolock(struct file *file, const struct iovec *iov,
 	 * exceeded without writing data we send a signal and return EFBIG.
 	 * Linus frestrict idea will clean these up nicely..
 	 */
-	if (likely(!S_ISBLK(inode->i_mode))) {
+	if (likely(!isblk)) {
 		if (unlikely(pos >= inode->i_sb->s_maxbytes)) {
 			if (count || pos > inode->i_sb->s_maxbytes) {
 				send_sig(SIGXFSZ, current, 0);
@@ -1667,7 +1667,7 @@ generic_file_write_nolock(struct file *file, const struct iovec *iov,
 					iov, pos, nr_segs);
 		if (written > 0) {
 			loff_t end = pos + written;
-			if (end > inode->i_size && !S_ISBLK(inode->i_mode)) {
+			if (end > inode->i_size && !isblk) {
 				inode->i_size = end;
 				mark_inode_dirty(inode);
 			}