ecryptfs: Use a folio throughout ecryptfs_read_folio()

ecryptfs: Use a folio throughout ecryptfs_read_folio()

Remove the conversion to a struct page.  Removes a few hidden calls to
compound_head().  Use 'err' instead of 'rc' for clarity.

Also remove the unnecessary call to ClearPageUptodate(); the uptodate
flag is already clear if this function is being called.  That lets us
switch to folio_end_read() which does one atomic flag operation instead
of two.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20241025190822.1319162-3-willy@infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>