Collapse get_node/get_node_nocheck into a single function that
returns NULL when the node is absent rather than abort()ing.
Callers that previously assumed non-NULL now check the return value
and either reply ESTALE/ENOMEM or fall through gracefully; a few
places where the node is load-bearing (e.g. unhash during remove)
use fatal::abort with a descriptive message.
Switch mutex_lock / mutex_unlock pairs to mutex_lockguard so early
returns on error paths no longer leak the lock, and add NULL-safety
around fuse_dh in readdir/readdir_plus, fuse_dirents_init error
returns, the getattr path when both name and fh are empty, and the
msgbuf allocation in fuse_lib_read.
Pull the new xstrdup/xmalloc wrappers from xalloc.hpp for the root
node name so the common-case OOM path terminates with a diagnostic
instead of a silent corrupted node.
Also guard fuse_session_destroy against a NULL session - called
from error-recovery paths.
The low-level init/destroy callbacks were passed a userdata pointer
that mergerfs never consumed - the caller threaded it through
fuse_lowlevel_new only to hand it back unchanged. Remove the
parameter from the ops struct, from fuse_lowlevel_new(_common),
and from the internal fuse_ll struct; update fuse_lib_init and
fuse_lib_destroy accordingly.
Also drop fuse_reply_ioctl_iov: no caller in-tree invokes it and
the public API is defined by this vendored libfuse fork.
