squashfs: Return the actual error from squashfs_read_folio()

Since we actually know what error happened, we can report it instead
of having the generic code return -EIO for pages that were unlocked
without being marked uptodate.  Also remove a test of PageError since
we have the return value at this point.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>