mtd: sm_ftl: Replace deprecated strncpy with sysfs_emit in sm_attr_show

mtd: sm_ftl: Replace deprecated strncpy with sysfs_emit in sm_attr_show

strncpy() is deprecated [1] for NUL-terminated destination buffers
because it does not guarantee NUL termination. It also unnecessarily
NUL-pads the destination buffer if the source is shorter. Replace it
with sysfs_emit() using the "%.*s" format specifier and supply the
length 'sm_attr->len' to improve sm_attr_show().

Return the number of characters actually written to 'buf' instead of
'sm_attr->len'.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>