hostfs: Use kasprintf() instead of fixed buffer formatting

hostfs: Use kasprintf() instead of fixed buffer formatting

BugLink: https://bugs.launchpad.net/bugs/1926489
[ Upstream commit b58c4e96192ee7c47d5c67853b1557306cfa0e7f ]

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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>