mds: use variable g_ceph_context directly in MDSAuthCaps

mds: use variable g_ceph_context directly in MDSAuthCaps

Variable g_ceph_context is global, therefore use it directly instead of
passing it as a parameter to method.

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit 64c2712fdfa9a524f7d3d6ac15660ed0585bfe6e)

Conflicts:
	src/mon/AuthMonitor.cc
	- Conflict happened due to absence of commit
	  3fcbb0bcf2bca58d6ca460ec036dc1a986c9875b