bcachefs: Fix bch2_extent_fallocate()

bcachefs: Fix bch2_extent_fallocate()

 - There was no need for a retry loop in bch2_extent_fallocate(); if we
   have to retry we may be overwriting something different and we need
   to return an error and let the caller retry.
 - The bch2_alloc_sectors_start() error path was wrong, and wasn't
   running our cleanup at the end of the function

This also fixes a very rare open bucket leak due to the missing cleanup.

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