debugfs: fix mount options not being applied

Mount options (uid, gid, mode) are silently ignored when debugfs is
mounted. This is a regression introduced during the conversion to the
new mount API.

When the mount API conversion was done, the parsed options were never
applied to the superblock when it was reused. As a result, the mount
options were ignored when debugfs was mounted.

Fix this by following the same pattern as the tracefs fix in commit
e4d32142d1de ("tracing: Fix tracefs mount options"). Call
debugfs_reconfigure() in debugfs_get_tree() to apply the mount options
to the superblock after it has been created or reused.

As an example, with the bug the "mode" mount option is ignored:

$ mount -o mode=0666 -t debugfs debugfs /tmp/debugfs_test
$ mount | grep debugfs_test
debugfs on /tmp/debugfs_test type debugfs (rw,relatime)
$ ls -ld /tmp/debugfs_test
drwx------ 25 root root 0 Aug 4 14:16 /tmp/debugfs_test

With the fix applied, it works as expected:

$ mount -o mode=0666 -t debugfs debugfs /tmp/debugfs_test
$ mount | grep debugfs_test
debugfs on /tmp/debugfs_test type debugfs (rw,relatime,mode=666)
$ ls -ld /tmp/debugfs_test
drw-rw-rw- 37 root root 0 Aug 2 17:28 /tmp/debugfs_test

Fixes: a20971c18752 ("vfs: Convert debugfs to use the new mount API")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220406
Cc: stable@vger.kernel.org
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
Link: https://lore.kernel.org/20250816-debugfs-mount-opts-v3-1-d271dad57b5b@posteo.net
Signed-off-by: Christian Brauner <brauner@kernel.org>