ANDROID: dma-buf: don't re-purpose kobject as work_struct

The commit 5aec776e ("BACKPORT: ANDROID: dma-buf: Move sysfs work
out of DMA-BUF export path) re-purposed kobject as work_struct temporarily
to create the sysfs entries asynchronously. The author knows what he is
doing and rightly added a build assert if kobject struct size is smaller than
the work_struct size. We are hitting this build assert on a non-GKI platform
where CONFIG_ANDROID_KABI_RESERVE is not set. Fix this problem by allocating
a new union with dma_buf_sysfs_entry structure and temporary structure as
members. We only end up allocating more memory (because of union) only when
kobject size is smaller than work_struct which the original patch any way
assumed would never be true.

Bug: 261818147
Change-Id: Ifb089bf80d8a3a44ece9f05fc0b99ee76cb11645
Signed-off-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>