fscrypt: switch from ->is_encrypted() to IS_ENCRYPTED()

IS_ENCRYPTED() now gives the same information as
i_sb->s_cop->is_encrypted() but is more efficient, since IS_ENCRYPTED()
is just a simple flag check.  Prepare to remove ->is_encrypted() by
switching all callers to IS_ENCRYPTED().

Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>