fuse: lock inode unconditionally in fuse_fallocate()

file_modified() must be called with inode lock held.  fuse_fallocate()
didn't lock the inode in case of just FALLOC_KEEP_SIZE flags value, which
resulted in a kernel Warning in notify_change().

Lock the inode unconditionally, like all other fallocate implementations
do.
Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Reported-and-tested-by: syzbot+462da39f0667b357c4b6@syzkaller.appspotmail.com
Fixes: 4a6f278d ("fuse: add file_modified() to fallocate")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -2963,11 +2963,9 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 		.mode = mode
 	};
 	int err;
-	bool lock_inode = !(mode & FALLOC_FL_KEEP_SIZE) ||
-			   (mode & (FALLOC_FL_PUNCH_HOLE |
-				    FALLOC_FL_ZERO_RANGE));
-
-	bool block_faults = FUSE_IS_DAX(inode) && lock_inode;
+	bool block_faults = FUSE_IS_DAX(inode) &&
+		(!(mode & FALLOC_FL_KEEP_SIZE) ||
+		 (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE)));
 
 	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE |
 		     FALLOC_FL_ZERO_RANGE))
@@ -2976,22 +2974,20 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	if (fm->fc->no_fallocate)
 		return -EOPNOTSUPP;
 
-	if (lock_inode) {
-		inode_lock(inode);
-		if (block_faults) {
-			filemap_invalidate_lock(inode->i_mapping);
-			err = fuse_dax_break_layouts(inode, 0, 0);
-			if (err)
-				goto out;
-		}
+	inode_lock(inode);
+	if (block_faults) {
+		filemap_invalidate_lock(inode->i_mapping);
+		err = fuse_dax_break_layouts(inode, 0, 0);
+		if (err)
+			goto out;
+	}
 
-		if (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE)) {
-			loff_t endbyte = offset + length - 1;
+	if (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE)) {
+		loff_t endbyte = offset + length - 1;
 
-			err = fuse_writeback_range(inode, offset, endbyte);
-			if (err)
-				goto out;
-		}
+		err = fuse_writeback_range(inode, offset, endbyte);
+		if (err)
+			goto out;
 	}
 
 	if (!(mode & FALLOC_FL_KEEP_SIZE) &&
@@ -3039,8 +3035,7 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	if (block_faults)
 		filemap_invalidate_unlock(inode->i_mapping);
 
-	if (lock_inode)
-		inode_unlock(inode);
+	inode_unlock(inode);
 
 	fuse_flush_time_update(inode);