Remove vn_set_fs_pwd()/vn_set_pwd() (no need to be at / during insmod)

Per suggestion from @behlendorf in #8777, remove vn_set_fs_pwd() and
vn_set_pwd() which are only used in zfs_ioctl.c:_init() while loading
zfs.ko.

The rest of initialization functions being called here after cwd set
to / don't depend on cwd of the process except for spa_config_load().
spa_config_load() uses a relative path ".//etc/zfs/zpool.cache" when
`rootdir` is non-NULL, which is "/etc/zfs/zpool.cache" given cwd is /,
so just unconditionally use the absolute path without "./", so that
`vn_set_pwd("/")` as well as the entire functions can be removed.
This is also what FreeBSD does.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #8826

config/kernel-spinlock.m4

@@ -1,24 +0,0 @@
-dnl #
-dnl # 2.6.36 API change,
-dnl # The 'struct fs_struct->lock' was changed from a rwlock_t to
-dnl # a spinlock_t to improve the fastpath performance.
-dnl #
-AC_DEFUN([ZFS_AC_KERNEL_FS_STRUCT_SPINLOCK], [
-	AC_MSG_CHECKING([whether struct fs_struct uses spinlock_t])
-	tmp_flags="$EXTRA_KCFLAGS"
-	EXTRA_KCFLAGS="-Werror"
-	ZFS_LINUX_TRY_COMPILE([
-		#include <linux/sched.h>
-		#include <linux/fs_struct.h>
-	],[
-		static struct fs_struct fs;
-		spin_lock_init(&fs.lock);
-	],[
-		AC_MSG_RESULT(yes)
-		AC_DEFINE(HAVE_FS_STRUCT_SPINLOCK, 1,
-		          [struct fs_struct uses spinlock_t])
-	],[
-		AC_MSG_RESULT(no)
-	])
-	EXTRA_KCFLAGS="$tmp_flags"
-])

config/kernel.m4

@@ -12,7 +12,6 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_CTL_NAME
 	ZFS_AC_KERNEL_PDE_DATA
 	ZFS_AC_KERNEL_2ARGS_VFS_FSYNC
-	ZFS_AC_KERNEL_FS_STRUCT_SPINLOCK
 	ZFS_AC_KERNEL_KUIDGID_T
 	ZFS_AC_KERNEL_FALLOCATE
 	ZFS_AC_KERNEL_2ARGS_ZLIB_DEFLATE_WORKSPACESIZE