target/file: Bump FD_MAX_SECTORS to 2048 to handle 1M sized I/Os

This patch bumps the default FILEIO backend FD_MAX_SECTORS value from
1024 -> 2048 in order to allow block_size=512 to handle 1M sized I/Os.

The current default rejects I/Os larger than 512K in sbc_parse_cdb():

[12015.915146] SCSI OP 2ah with too big sectors 1347 exceeds backend
hw_max_sectors: 1024
[12015.977744] SCSI OP 2ah with too big sectors 2048 exceeds backend
hw_max_sectors: 1024

This issue is present in >= v3.5 based kernels, introduced after the
removal of se_task logic.

Reported-by: Viljami Ilola <azmulx@netikka.fi>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>