fscrypt: introduce fscrypt_needs_contents_encryption()

fscrypt: introduce fscrypt_needs_contents_encryption()

Add a function fscrypt_needs_contents_encryption() which takes an inode
and returns true if it's an encrypted regular file and the kernel was
built with fscrypt support.

This will allow replacing duplicated checks of IS_ENCRYPTED() &&
S_ISREG() on the I/O paths in ext4 and f2fs, while also optimizing out
unneeded code when !CONFIG_FS_ENCRYPTION.

Link: https://lore.kernel.org/r/20191209205021.231767-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>