client: cast m->get_client_tid() to compare to 16-bit Inode::flushing_cap_tid

m->get_client_tid() is 64 bits (as it should be), but Inode::flushing_cap_tid
is only 16 bits. 16 bits should be plenty to let the cap flush updates
pipeline appropriately, but we need to cast in the proper direction when
comparing these differently-sized versions. So downcast the 64-bit one
to 16 bits.

Fixes: #9869
Backport: giant, firefly, dumpling

Signed-off-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit a5184cf46a6e867287e24aeb731634828467cd98)
Merge pull request #2840 from ceph/firefly-9869

Backport "client: cast m->get_client_tid() to compare to 16-bit Inode::flushing_cap_tid"