fs/exfat: fix every bio only has one block

fs/exfat: fix every bio only has one block

This patch fix the bug that exfat fs cannot write
continuous blocks in one bio.
This will cause every request submit in block layer
only contains one block, cause write speed slower.

Change-Id: Ia548a7d8051defc1498d2d11d39e069bf683fdff
Signed-off-by: haojl2 <haojl2@motorola.com>
Signed-off-by: Shiyong Li <a22381@motorola.com>
Reviewed-on: https://gerrit.mot.com/1102183
SLTApproved: Slta Waiver
SME-Granted: SME Approvals Granted
Tested-by: Jira Key
Reviewed-by: Igor Kovalenko <igork@motorola.com>
Submit-Approved: Jira Key
Reviewed-on: https://gerrit.mot.com/1276884
Reviewed-by: Hua Tan <tanhua1@motorola.com>