ANDROID: fuse: Allocate zeroed memory for canonical path

The page used to contain the fuse_dentry_canonical_path to be handled in
fuse_dev_do_write is allocated using __get_free_pages(GFP_KERNEL).
The returned page may contain undefined data, that by chance may be
considered as a valid path name that is not in the cache. In that case,
if the FUSE daemon mistakenly doesn't fill the canonical path buffer,
the FUSE driver may fall into two blocking

  request_wait_answer(fuse_dev_write->kern_path->fuse_lookup_name)

causing a deadlock condition.

The stack is as follows：
find            S    0 20511  20117 0x00000000
Call trace:
[<ffffff8008085e78>] __switch_to+0xb8/0xd4
[<ffffff8008a0cac4>] __schedule+0x458/0x714
[<ffffff8008a0ce0c>] schedule+0x8c/0xa8
[<ffffff800833865c>] request_wait_answer+0x74/0x220
[<ffffff8008339f70>] __fuse_request_send+0x8c/0xa0
[<ffffff8008339fe4>] fuse_request_send+0x60/0x6c
[<ffffff800833c1a8>] fuse_dentry_canonical_path+0xb8/0x104
[<ffffff800820b14c>] do_sys_open+0x1b4/0x260
[<ffffff800820b27c>] SyS_openat+0x3c/0x4c
[<ffffff8008083540>] el0_svc_naked+0x34/0x38
mount.ntfs-3g   S    0  5845      1 0x00000000
Call trace:
[<ffffff8008085e78>] __switch_to+0xb8/0xd4
[<ffffff8008a0cac4>] __schedule+0x458/0x714
[<ffffff8008a0ce0c>] schedule+0x8c/0xa8
[<ffffff800833865c>] request_wait_answer+0x74/0x220
[<ffffff8008339f70>] __fuse_request_send+0x8c/0xa0
[<ffffff8008339fe4>] fuse_request_send+0x60/0x6c
[<ffffff800833bdb0>] fuse_simple_request+0x128/0x16c
[<ffffff800833dddc>] fuse_lookup_name+0x104/0x1b0
[<ffffff800833dee4>] fuse_lookup+0x5c/0x11c
[<ffffff800821861c>] lookup_slow+0xfc/0x174
[<ffffff800821b474>] walk_component+0xf0/0x290
[<ffffff800821bbac>] path_lookupat+0xa0/0x128
[<ffffff800821c7f4>] filename_lookup+0x84/0x124
[<ffffff800821c8d8>] kern_path+0x44/0x54
[<ffffff800833b0c8>] fuse_dev_do_write+0x828/0xa0c
[<ffffff800833b610>] fuse_dev_write+0x90/0xb4
[<ffffff800820b770>] do_iter_readv_writev+0xf4/0x13c
[<ffffff800820cc88>] do_readv_writev+0xec/0x220
[<ffffff800820d05c>] vfs_writev+0x60/0x74
[<ffffff800820d0ec>] do_writev+0x7c/0x100
[<ffffff800820e348>] SyS_writev+0x38/0x48
[<ffffff8008083540>] el0_svc_naked+0x34/0x38

Fix by ensuring that the page allocated for the canonical path is zeroed.

Bug: 194856119
Bug: 196051870
Fixes: 24ab59f6bb42 ("ANDROID: fuse: Add support for d_canonical_path")
Signed-off-by: Biao Li <libiao@allwinnertech.com>
Signed-off-by: Shuosheng Huang <huangshuosheng@allwinnertech.com>
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: I400815dc1049d90c308f5cf87ce60de97ff82131