fs: ext4: constify the buffer passed to write functions

fs: ext4: constify the buffer passed to write functions

There is no need to modify the buffer passed to ext4fs_write_file().
The memset() call is not required here and was likely copied from the
equivalent part of the ext4fs_read_file() function where we do need it.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>