mds: tidy up backtrace pool handling

mds: tidy up backtrace pool handling

Include the pool ID in the log message when
we get a write error (and don't say "dir ino"
because this is also the path for files)

Move the duplicated logic for picking the pool out
into get_backtrace_pool()

In get_backtrace_pool(), assert that files do indeed
have a pool set.

Signed-off-by: John Spray <john.spray@redhat.com>