client: fix Dentry UAF by holding DentryRef across unlink

Hold a function-scoped DentryRef in Client::unlink() to keep the dentry
alive through the entire teardown sequence.

Without the O_DIRECTORY dentry pin (PR #60909, merged after v19.2.1),
opening a directory does not call _ll_get(), so the dentry stays at
ref=1 while inode->dir is still active. When trim_cache() evicts this
dentry, Dentry::unlink() calls put() for the dir pin, ref drops to 0,
and the dentry is freed while Client::unlink() still needs it for
detach/lru_remove.

A function-scoped DentryRef guarantees the dentry survives until after
it has been properly removed from the dir and LRU, regardless of pin
state.

Signed-off-by: Igor Golikov <igolikov@ibm.com>
Fixes: https://tracker.ceph.com/issues/74625