isofs: Convert rock_ridge_symlink_read_folio to use a folio

isofs: Convert rock_ridge_symlink_read_folio to use a folio

Remove the conversion back into a page and use the folio APIs throughout.
Remove the setting of PG_error instead of converting it; it is unused
by core code or by the rest of isofs, so it serves no purpose here.
Use folio_end_read() to save an atomic operation and unify the two
exit paths.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20240530202110.2653630-7-willy@infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>