Fix zpl_test_super race with zfs_umount

We cannot call zpl_enter in zpl_test_super, because zpl_test_super is
under spinlock so we can't sleep, and also because zpl_test_super is
called without sb->s_umount taken, so it's possible we would race with
zfs_umount and call zpl_enter on freed zfsvfs.

Here's an stack trace when this happens:
[ 2379.114837] VERIFY(cvp->cv_magic == CV_MAGIC) failed
[ 2379.114845] PANIC at spl-condvar.c:497:cv_broadcast()
[ 2379.114854] Kernel panic - not syncing: VERIFY(cvp->cv_magic == CV_MAGIC) failed
[ 2379.115012] Call Trace:
[ 2379.115019] dump_stack+0x74/0x96
[ 2379.115024] panic+0x114/0x2f6
[ 2379.115035] spl_panic+0xcf/0xfc [spl]
[ 2379.115477] cv_broadcast+0x68/0xa0 [spl]
[ 2379.115585] rrw_exit+0xb8/0x310 [zfs]
[ 2379.115696] rrm_exit+0x4a/0x80 [zfs]
[ 2379.115808] zpl_test_super+0xa9/0xd0 [zfs]
[ 2379.115920] sget+0xd1/0x230
[ 2379.116033] zpl_mount+0xdc/0x230 [zfs]
[ 2379.116037] legacy_get_tree+0x28/0x50
[ 2379.116039] vfs_get_tree+0x27/0xc0
[ 2379.116045] path_mount+0x2fe/0xa70
[ 2379.116048] do_mount+0x80/0xa0
[ 2379.116050] __x64_sys_mount+0x8b/0xe0
[ 2379.116052] do_syscall_64+0x35/0x50
[ 2379.116054] entry_SYSCALL_64_after_hwframe+0x61/0xc6
[ 2379.116057] RIP: 0033:0x7f9912e8b26a

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #15077