fscrypt: document gfp_flags for bounce page allocation

Document that fscrypt_encrypt_pagecache_blocks() allocates the bounce
page from a mempool, and document what this means for the @gfp_flags
argument.

Link: https://lore.kernel.org/r/20191231181026.47400-1-ebiggers@kernel.org


Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>