For unmapped range, the returned map.m_llen is zero, and thus the
calculated count is unexpected zero.
Prior to the refactoring introduced by commit 1ae9470c3e14 ("erofs:
clean up .read_folio() and .readahead() in fscache mode"), only the
readahead routine suffers from this. With the refactoring of making
.read_folio() and .readahead() calling one common routine, both
read_folio and readahead have this issue now.
Fix this by calculating count separately in unmapped condition.
Fixes: c665b394b9e8 ("erofs: implement fscache-based data readahead")
Fixes: 1ae9470c3e14 ("erofs: clean up .read_folio() and .readahead() in fscache mode")
Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>