hostfs: Use kasprintf() instead of fixed buffer formatting

[ Upstream commit b58c4e96

 ]

Improve readability and maintainability by replacing a hardcoded string
allocation and formatting by the use of the kasprintf() helper.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>