Fix a declaration position of the nth_page.

Compilation time bug introduced by 87df5e4 commit.
Fix for the compilation error(Linux kernel 6.18.0):
"zfs/module/os/linux/zfs/abd_os.c:920:32: error: implicit declaration
of function ‘nth_page’; did you mean ‘pte_page’?
[-Werror=implicit-function-declaration]".

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: agiUnderground <alex.dev.cv@gmail.com>
Closes #18034