ANDROID: pstore: Use vmalloc for large allocations due to ramoops size

Android uses a 1 MiB console ramoops region, which requires kmalloc
to be changed to vmalloc in the following places:

1. pstore_mkfile(), allocation of inode->i_private
2. ramoops_pstore_read(), allocation of buf

Bug: 67383905
Change-Id: Ie4f355a5991b7cb6ad356ded7bd9d41630602bf5
Signed-off-by: Siqi Lin <siqilin@google.com>