fscrypt: updates on 4.15-rc4

Cherry-picked from origin/upstream-f2fs-stable-linux-4.9.y:

743205fb fscrypt: move to generic async completion
f1eb0c0b crypto: introduce crypto wait for async op
e0af083a fscrypt: lock mutex before checking for bounce page pool
9e48a9fd fscrypt: new helper function - fscrypt_prepare_setattr()
ec822ff8 fscrypt: new helper function - fscrypt_prepare_lookup()
98fe83a1 fscrypt: new helper function - fscrypt_prepare_rename()
f5218702 fscrypt: new helper function - fscrypt_prepare_link()
d61dffbd fscrypt: new helper function - fscrypt_file_open()
5190ed07 fscrypt: new helper function - fscrypt_require_key()
8814204a fscrypt: remove unneeded empty fscrypt_operations structs
8745aa36 fscrypt: remove ->is_encrypted()
d750ec72 fscrypt: switch from ->is_encrypted() to IS_ENCRYPTED()
685285b0 fs, fscrypt: add an S_ENCRYPTED inode flag
1617929c fscrypt: clean up include file mess
a0471ef4 fscrypt: fix dereference of NULL user_key_payload
e77e7df0 fscrypt: make ->dummy_context() return bool

Change-Id: I23f36bfd059c0c576608221e7e1135535646cc5d
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>