bcachefs: Use DECLARE_FLEX_ARRAY() helper and fix multiple -Warray-bounds warnings

Transform zero-length array `s` into a proper flexible-array
member in `struct snapshot_table` via the DECLARE_FLEX_ARRAY()
helper; and fix tons of the following -Warray-bounds warnings:

fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
fs/bcachefs/snapshot.c:135:70: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]
fs/bcachefs/snapshot.h:36:21: warning: array subscript <unknown> is outside array bounds of 'struct snapshot_t[0]' [-Warray-bounds=]

This helps with the ongoing efforts to globally enable -Warray-bounds.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>