Use scrypt from libsodium

Use scrypt from libsodium

bcachefs-tools has both libscrypt and libsodium as build dependencies,
but libsodium already includes the same scrypt implementation as libscrypt,
originally written by Colin Percival.

Use the libsodium copy, dropping the extra libscrypt dependency.

Explicitly adopt the default scrypt N, r and p values from libscrypt to
avoid unintended changes in the default work parameters for bcachefs.

Signed-off-by: Chris Webb <chris@arachsys.com>