sysctl: Create pipe-max-size converter using sysctl UINT macros

sysctl: Create pipe-max-size converter using sysctl UINT macros

Create a converter for the pipe-max-size proc_handler using the
SYSCTL_UINT_CONV_CUSTOM. Move SYSCTL_CONV_IDENTITY macro to the sysctl
header to make it available for pipe size validation. Keep returning
-EINVAL when (val == 0) by using a range checking converter and setting
the minimal valid value (extern1) to SYSCTL_ONE. Keep round_pipe_size by
passing it as the operation for SYSCTL_USER_TO_KERN_INT_CONV.

Signed-off-by: Joel Granados <joel.granados@kernel.org>