fscrypt: updates on 4.15-rc4

Cherry-picked from origin/upstream-f2fs-stable-linux-3.18.y:

f73a1ef7 fscrypt: resolve some cherry-pick bugs
b9dad2c0 fscrypt: move to generic async completion
fb0097fe crypto: introduce crypto wait for async op
89682e2b fscrypt: lock mutex before checking for bounce page pool
16e02cc0 fscrypt: new helper function - fscrypt_prepare_setattr()
03c10b3f fscrypt: new helper function - fscrypt_prepare_lookup()
95b5e0ec fscrypt: new helper function - fscrypt_prepare_rename()
1b4465a3 fscrypt: new helper function - fscrypt_prepare_link()
25c01cbc fscrypt: new helper function - fscrypt_file_open()
63aa0239 fscrypt: new helper function - fscrypt_require_key()
7fa20786 fscrypt: remove unneeded empty fscrypt_operations structs
7319c5ee fscrypt: remove ->is_encrypted()
d824e3a7 fscrypt: switch from ->is_encrypted() to IS_ENCRYPTED()
12f169fd fs, fscrypt: add an S_ENCRYPTED inode flag
0f461ce2 fscrypt: clean up include file mess
db32474d fscrypt: fix dereference of NULL user_key_payload
12cff7c6 fscrypt: make ->dummy_context() return bool

Change-Id: Iaf8f427637741c70f529a7571f9376695d441f1a
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>