ext4: make ext4_ext_get_blocks always return <= max_blocks

ext4_ext_get_blocks() returns number of blocks allocated with buffer
heads unmapped for a read from prealloc space.  This is needed so that
delayed allocation doesn't do block reservation for prealloc space since
the blocks are already resevred on disk.  Fix ext4_ext_get_blocks to not
return greater than max_blocks, since some of the code paths cannot
handle such a return value.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -2570,8 +2570,18 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
 			}
 			if (create == EXT4_CREATE_UNINITIALIZED_EXT)
 				goto out;
-			if (!create)
+			if (!create) {
+				/*
+				 * We have blocks reserved already.  We
+				 * return allocated blocks so that delalloc
+				 * won't do block reservation for us.  But
+				 * the buffer head will be unmapped so that
+				 * a read from the block returns 0s.
+				 */
+				if (allocated > max_blocks)
+					allocated = max_blocks;
 				goto out2;
+			}
 
 			ret = ext4_ext_convert_to_initialized(handle, inode,
 								path, iblock,