bcachefs: Fix multiple -Warray-bounds warnings

Transform zero-length array `entries` into a proper flexible-array
member in `struct journal_seq_blacklist_table`; and fix the following
-Warray-bounds warnings:

fs/bcachefs/journal_seq_blacklist.c:148:26: warning: array subscript idx is outside array bounds of 'struct journal_seq_blacklist_table_entry[0]' [-Warray-bounds=]
fs/bcachefs/journal_seq_blacklist.c:150:30: warning: array subscript idx is outside array bounds of 'struct journal_seq_blacklist_table_entry[0]' [-Warray-bounds=]
fs/bcachefs/journal_seq_blacklist.c:154:27: warning: array subscript idx is outside array bounds of 'struct journal_seq_blacklist_table_entry[0]' [-Warray-bounds=]
fs/bcachefs/journal_seq_blacklist.c:176:27: warning: array subscript i is outside array bounds of 'struct journal_seq_blacklist_table_entry[0]' [-Warray-bounds=]
fs/bcachefs/journal_seq_blacklist.c:177:27: warning: array subscript i is outside array bounds of 'struct journal_seq_blacklist_table_entry[0]' [-Warray-bounds=]
fs/bcachefs/journal_seq_blacklist.c:297:34: warning: array subscript i is outside array bounds of 'struct journal_seq_blacklist_table_entry[0]' [-Warray-bounds=]
fs/bcachefs/journal_seq_blacklist.c:298:34: warning: array subscript i is outside array bounds of 'struct journal_seq_blacklist_table_entry[0]' [-Warray-bounds=]
fs/bcachefs/journal_seq_blacklist.c:300:31: warning: array subscript i is outside array bounds of 'struct journal_seq_blacklist_table_entry[0]' [-Warray-bounds=]

This results in no differences in binary output.

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>