hostfs: Fix the NULL vs IS_ERR() bug for __filemap_get_folio()

hostfs: Fix the NULL vs IS_ERR() bug for __filemap_get_folio()

[ Upstream commit bed2cc482600296fe04edbc38005ba2851449c10 ]

The __filemap_get_folio() function returns error pointers.
It never returns NULL. So use IS_ERR() to check it.

Fixes: 1da86618bdce ("fs: Convert aops->write_begin to take a folio")
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>