bcachefs: Free journal bufs when not in use

bcachefs: Free journal bufs when not in use

Since we're increasing the number of 'struct journal_bufs', we don't
want them all permanently holding onto buffers for the journal data -
that'd be 16 * 2MB = 32MB, or potentially more.

Add a single-element mempool (open coded, since buffer size varies),
this also means we won't be hitting the memory allocator every time we
open and close a journal entry/buffer.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>