ANDROID: sdcardfs: evict dentries on fscrypt key removal

Use the fscrypt key removal notifier chain to make sdcardfs evict its
dentries when an fscrypt key is about to be removed.  This is needed for
the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl to properly "lock" the encrypted
files underneath sdcardfs when an Android user is stopped.

Test: pm create-user 10
      am start-user 10
      find /data/media/10/    # filenames are in plaintext form
      am stop-user 10
      find /data/media/10/    # filenames are in ciphertext form

      (But currently the kernel and vold still warn about other files
      still being open, due to b/140762419)

Bug: 120446149
Bug: 142275883
Change-Id: I83b451a2bc40c72fcd01d24aa5c34ad8de427534
Signed-off-by: Eric Biggers <ebiggers@google.com>