core dump in GPFSFAL_start_grace if there are no exports

Signed-off-by: Rick Mesta <rmesta@us.ibm.com>
1. cache_inode_lookup should return success if file exists.
2. cache_inode_create needs to lookup file again after create to
   handle race condition.

Author: Amit Dror <amitd@infinidat.com>
Signed-off-by: Rong Zeng <rongzeng@us.ibm.com>