ufs: fix ufs_read_cylinder() failure handling

ufs: fix ufs_read_cylinder() failure handling

1) ufs_load_cylinder() should return NULL on ufs_read_cylinder() failures.
ufs_error() is not enough.  As it is, IO failure on attempt to read a part
of cylinder group metadata is likely to end up with an oops.

2) we drop the wrong buffer heads when undoing sb_bread() on IO failure
halfway through the read - we need to brelse() what we've got from
sb_bread(), TYVM...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>