nilfs2: Remove check for PageError

[ Upstream commit 79ea6556 ]

If read_mapping_page() encounters an error, it returns an errno, not a
page with PageError set, so this test is not needed.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Stable-dep-of: 7373a51e ("nilfs2: fix nilfs_empty_dir() misjudgment and long loop on I/O errors")
Signed-off-by: Sasha Levin <sashal@kernel.org>