mds/MDSAuthCaps: allow adding multiple caps via "fs auth" cmd

mds/MDSAuthCaps: allow adding multiple caps via "fs auth" cmd

When multiple caps are passed to "ceph fs authorize" command, the
command hangs because the MON crashes due to this command. Underlying
code assumes that only one cap can be passed at a time by the user
through the "ceph fs authorize" command. "ceph_assert()" checks if only
one cap is passed. If not, the code crashes.

Fixes: https://tracker.ceph.com/issues/64127
Signed-off-by: Rishabh Dave <ridave@redhat.com>