ANDROID: fscrypt: add key removal notifier chain

Add a notifier chain so that sdcardfs can 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.

This is meant to be a temporary patch carried as part of the sdcardfs
patchset until either we stop using sdcardfs, we get sdcardfs upstream,
or we find a way to provide what sdcardfs needs while also benefitting a
user upstream.

Bug: 120446149
Bug: 142275883
Test: see I83b451a2bc40c72fcd01d24aa5c34ad8de427534
Change-Id: Iec79775a71057d05a371d77da4a6541cb8e09cb7
Signed-off-by: Eric Biggers <ebiggers@google.com>