ext4: if zeroout fails fall back to splitting the extent node

If the underlying storage device is using thin-provisioning, it's
possible for a zeroout operation to return ENOSPC.

Commit df22291ff0fd ("ext4: Retry block allocation if we have free blocks
left") added logic to retry block allocation since we might get free block
after we commit a transaction. But the ENOSPC from thin-provisioning
will confuse ext4, and lead to an infinite loop.

Since using zeroout instead of splitting the extent node is an
optimization, if it fails, we might as well fall back to splitting the
extent node.

Reported-by: yangerkun <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>

fs/ext4/extents.c

···
 				split_map.m_len - ee_block);
 			err = ext4_ext_zeroout(inode, &zero_ex1);
 			if (err)
-				goto out;
+				goto fallback;
 			split_map.m_len = allocated;
 		}
 		if (split_map.m_lblk - ee_block + split_map.m_len <
···
 						      ext4_ext_pblock(ex));
 				err = ext4_ext_zeroout(inode, &zero_ex2);
 				if (err)
-					goto out;
+					goto fallback;
 			}
 
 			split_map.m_len += split_map.m_lblk - ee_block;
···
 		}
 	}
 
+fallback:
 	err = ext4_split_extent(handle, inode, ppath, &split_map, split_flag,
 				flags);
 	if (err > 0)