mds: check snaprealm before drop dentries in deleted directory

mds: check snaprealm before drop dentries in deleted directory

There is an optimization that drop dentries in deleted directory,
so that MDS can purge corresponding inode faster. We need to check
if the deleted directory is still referenced by any snapshot. If
it is, we can't drop dentries or skip committing dirfrags to object
store. The simplest method to check if a delted directory is
potentially referenced by snaphosts is checking diri->snaprealm.

Signed-off-by: Yan, Zheng <zyan@redhat.com>