tracefs: Use dentry name snapshots instead of heap allocation

In fs/tracefs/inode.c, tracefs_syscall_mkdir() and tracefs_syscall_rmdir()
previously used a local helper, get_dname(), which allocated a temporary
buffer on the heap via kmalloc() to hold the dentry name. This introduced
unnecessary overhead, an ENOMEM failure path, and required manual memory
cleanup via kfree().

As suggested by Al Viro, replace this heap allocation with the VFS dentry
name snapshot API. By stack-allocating a `struct name_snapshot` and using
take_dentry_name_snapshot() and release_dentry_name_snapshot(), we safely
capture the dentry name locklessly, eliminate the heap allocation entirely,
and remove the now-obsolete error handling paths. The get_dname() helper
is completely removed.

Testing:
Booted a custom kernel natively in virtme-ng (ARM64). Triggered tracefs
inode and dentry allocation by creating and removing a custom directory
under a temporary tracefs mount. Verified that the instance is created
successfully and that no memory errors or warnings are emitted in dmesg.

Link: https://patch.msgid.link/20260306200458.2264-1-anishm7030@gmail.com
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: AnishMulay <anishm7030@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

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