res_fax: Fix deadlock in ast_channel_get_t38_state().

ast_channel_get_t38_state() calls ast_channel_queryoption() with
AST_OPTION_T38_STATE.  If the passed in channel is a local channel then a
deadlock can happen if a channel lock is held when called.

* Made ast_channel_get_t38_state() callers not hold a channel lock before
calling.

* Update ast_channel_get_t38_state() doxygen to note that no channel locks
can be held when calling the function.

ASTERISK-26203 #close
Reported by: Etienne Lessard

ASTERISK-24822 #close
Reported by: David Brillert

ASTERISK-22732 #close
Reported by: Richard Mudgett

Change-Id: I49fd76fa9af628b4198009b5c0b82c8b03681214