os: fallback for unsupported posix_fallocate

Some filesystems do not support fallocate and return EOPNOTSUPP.

On musl-based distros libwayland-cursor exits abruptly which causes the
application to crash. Unlike glibc, musl does not provide a fallback
mechanism for handling unsupported fallocate. Instead, musl developers
argue that application should handle the case of unsupported system
call.

This commit allows falback to ftruncate in case when EOPNOTSUPP
was recieved.

Signed-off-by: Ihor Antonov <ihor@antonovs.family>