fscrypt: updates on 4.15-rc4

Cherry-picked from origin/upstream-f2fs-stable-linux-4.4.y:

ba1ade71 fscrypt: resolve some cherry-pick bugs
9e32f17d fscrypt: move to generic async completion
4ecacbed crypto: introduce crypto wait for async op
42d89da8 fscrypt: lock mutex before checking for bounce page pool
2286508d fscrypt: new helper function - fscrypt_prepare_setattr()
5cbdd42a fscrypt: new helper function - fscrypt_prepare_lookup()
a31feba5 fscrypt: new helper function - fscrypt_prepare_rename()
95efafb6 fscrypt: new helper function - fscrypt_prepare_link()
2b4b4f98 fscrypt: new helper function - fscrypt_file_open()
8c815f38 fscrypt: new helper function - fscrypt_require_key()
272e4350 fscrypt: remove unneeded empty fscrypt_operations structs
1034eeec fscrypt: remove ->is_encrypted()
32c0d3ae fscrypt: switch from ->is_encrypted() to IS_ENCRYPTED()
a4781dd1 fs, fscrypt: add an S_ENCRYPTED inode flag
ff0a3dbc fscrypt: clean up include file mess
bc4a61c6 fscrypt: fix dereference of NULL user_key_payload
a53dc7e0 fscrypt: make ->dummy_context() return bool

Change-Id: I461d742adc7b77177df91429a1fd9c8624a698d6
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>