btrfs: fix compiler warning on SPARC/PA-RISC handling fscrypt_setup_filename

Commit 1ec49744 ("btrfs: turn on -Wmaybe-uninitialized") exposed
that on SPARC and PA-RISC, gcc is unaware that fscrypt_setup_filename()
only returns negative error values or 0. This ultimately results in a
maybe-uninitialized warning in btrfs_lookup_dentry().

Change to only return negative error values or 0 from
fscrypt_setup_filename() at the relevant call site, and assert that no
positive error codes are returned (which would have wider implications
involving other users).

Reported-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/all/481b19b5-83a0-4793-b4fd-194ad7b978c3@roeck-us.net/


Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>