mds: generate symlink inode with correct mode

mds: generate symlink inode with correct mode

Fix test instance generation for InodeStoreBare and InodeStore to
properly set the mode field to S_IFLNK for symlink inodes.

Previously, generated test instances with symlink inodes had unset
mode fields, creating inconsistent data. This issue was masked because
ceph-dencoder reused existing instances during encode/decode consistency
tests, leaving stale values intact.

The problem would surface when check-generated.sh and readable.sh
allocate fresh instances for decoding tests, as the missing mode field
would cause decode/encode inconsistencies.

This change fixes generate_test_instances() to set the mode field to
S_IFLNK for symlink inodes, creating valid InodeStore and InodeStoreBare
instances with consistent field values for proper encode/decode testing.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>