Micro-optimize __load_ioengine()

Micro-optimize __load_ioengine()

Instead of copying the I/O engine name, use a pointer to the I/O engine
name. This patch suppresses the following false positive Valgrind complaint:

Conditional jump or move depends on uninitialised value(s)
   at 0x41D451: __load_ioengine (ioengines.c:133)
   by 0x41D451: load_ioengine (ioengines.c:161)
   by 0x41FA59: ioengine_load (init.c:1126)
   by 0x423B64: parse_cmd_line (init.c:2670)
   by 0x4241A3: parse_options (init.c:2965)
   by 0x40F587: main (fio.c:42)

The code for copying I/O engine names was introduced by commit 2866c82d598e
("[PATCH] Separate io engines into separate loadable objects").

Signed-off-by: Bart Van Assche <bvanassche@acm.org>