sysctl: Indicate the direction of operation with macro names

sysctl: Indicate the direction of operation with macro names

Replace the "write" integer parameter with SYSCTL_USER_TO_KERN() and
SYSCTL_KERN_TO_USER() that clearly indicate data flow direction in
sysctl operations.

"write" originates in proc_sysctl.c (proc_sys_{read,write}) and can take
one of two values: "0" or "1" when called from proc_sys_read and
proc_sys_write respectively. When write has a value of zero, data is
"written" to a user space buffer from a kernel variable (usually
ctl_table->data). Whereas when write has a value greater than zero, data
is "written" to an internal kernel variable from a user space buffer.
Remove this ambiguity by introducing macros that clearly indicate the
direction of the "write".

The write mode names in sysctl_writes_mode are left unchanged as these
directly relate to the sysctl_write_strict file in /proc/sys where the
word "write" unambiguously refers to writing to a file.

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