mds/MDCache.cc: init 'default_file_layout' in ctor
Fix for:
CID 717237 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
uninit_member: Non-static class member field default_file_layout.fl_pg_pool
is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member field default_file_layout.fl_stripe_unit
is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member field default_file_layout.fl_stripe_count
is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member field default_file_layout.fl_object_size
is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member field default_file_layout.fl_cas_hash
is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member field default_file_layout.fl_object_stripe_unit
is not initialized in this constructor nor in any functions that it calls.
uninit_member: Non-static class member field default_file_layout.fl_unused
is not initialized in this constructor nor in any functions that it calls.
Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>