fscrypt: remove filesystem specific build config option

In order to have a common code base for fscrypt "post read" processing
for all filesystems which support encryption, this commit removes
filesystem specific build config option (e.g. CONFIG_EXT4_FS_ENCRYPTION)
and replaces it with a build option (i.e. CONFIG_FS_ENCRYPTION) whose
value affects all the filesystems making use of fscrypt.

Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/Kconfig

@@ -1,5 +1,5 @@
 config FS_ENCRYPTION
-	tristate "FS Encryption (Per-file encryption)"
+	bool "FS Encryption (Per-file encryption)"
 	select CRYPTO
 	select CRYPTO_AES
 	select CRYPTO_CBC
@@ -12,4 +12,5 @@ config FS_ENCRYPTION
 	  Enable encryption of files and directories.  This
 	  feature is similar to ecryptfs, but it is more memory
 	  efficient since it avoids caching the encrypted and
-	  decrypted pages in the page cache.
+	  decrypted pages in the page cache.  Currently Ext4,
+	  F2FS and UBIFS make use of this feature.

fs/crypto/fscrypt_private.h

@@ -12,7 +12,6 @@
 #ifndef _FSCRYPT_PRIVATE_H
 #define _FSCRYPT_PRIVATE_H
 
-#define __FS_HAS_ENCRYPTION 1
 #include <linux/fscrypt.h>
 #include <crypto/hash.h>