kernfs: Convert kernfs_walk_ns() from strlcpy() to strscpy()

kernfs: Convert kernfs_walk_ns() from strlcpy() to strscpy()

strlcpy() reads the entire source buffer first. This read may exceed
the destination size limit. This is both inefficient and can lead
to linear read overflows if a source string is not NUL-terminated[1].
Additionally, it returns the size of the source string, not the
resulting size of the destination string. In an effort to remove strlcpy()
completely[2], replace strlcpy() here with strscpy().

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
Link: https://github.com/KSPP/linux/issues/89
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Azeem Shaikh <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>