MDEV-31049 fil_delete_tablespace() returns wrong file handle if tablespace was closed by parallel thread

fil_delete_tablespace() stores file handle in local variable and calls
mtr_t::commit_file()=>fil_system_t::detach(..., detach_handle=true), which
sets space->chain.start->handle = OS_FILE_CLOSED. fil_system_t::detach()
is invoked under fil_system.mutex.

But before the mutex is acquired some parallel thread can change
space->chain.start->handle. fil_delete_tablespace() returns value, stored
in local variable, i.e. wrong value.

File handle can be closed, for example, from buf_flush_space() when the
limit of innodb_open_files exceded and fil_space_t::get() causes
fil_space_t::try_to_close() call.

fil_space_t::try_to_close() is executed under fil_system.mutex. And
mtr_t::commit_file() locks it for fil_system_t::detach() call.
fil_system_t::detach() returns detached file handle if its argument
detach_handle is true. The fix is to let mtr_t::commit_file() to pass
that detached file handle to fil_delete_tablespace().