filesetup: improve LFSR init failure error message

filesetup: improve LFSR init failure error message

Especially with small sample spaces, the LFSR random generator
occasionally fails to initialize successfully. When this occurs, the
error message refers to problems allocating a random map. Change the
error message to explicitly mention the LFSR failure.

OLD BEHAVIOR
$ ./fio --name=test --ioengine=null --size=4k --random_generator=lfsr --rw=randread --randrepeat=0
test: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=null, iodepth=1
fio-3.16-13-g998b-dirty
Starting 1 process
fio: failed allocating random map. If running a large number of jobs, try the 'norandommap' option or set 'softrandommap'. Or give a larger --alloc-size to fio.

NEW BEHAVIOR
$ ./fio --name=test --ioengine=null --size=4k --random_generator=lfsr --rw=randread --randrepeat=0
test: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=null, iodepth=1
fio-3.16-37-g65ee
Starting 1 process
fio: failed initializing LFSR

Signed-off-by: Jens Axboe <axboe@kernel.dk>