ZFS v28: kernel panics while reading an extended attribute

Hi!

It seems, I've found a bug in the ZFS v28 on the latest stable:
if we have a snapshot with some files having an extended attributes,
then attempt to read an extended attributes's value leads to a well
reproducible kernel panic.

The part of backtrace follows:

#6  0xffffffff804bbe44 in calltrap ()
    at /usr/src/sys/amd64/amd64/exception.S:228
#7  0xffffffff80950ea7 in zil_commit (zilog=0x0, foid=5795917)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zil.c:1497
#8  0xffffffff80979e6b in zfs_freebsd_read (ap=Variable "ap" is not available.)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c:622
#9  0xffffffff80979750 in zfs_getextattr (ap=0xffffff80dd5d8820)
    at vnode_if.h:384
#10 0xffffffff8038921b in extattr_get_vp (vp=0xffffff0056a01588,
    attrnamespace=1, attrname=0xffffff80dd5d89a0 "DOSATTRIB", data=Variable "data" is not available.)
    at vnode_if.h:1332

It seems that ZIL isn't available for snapshots, but zfs_freebsd_read
doesn't check this when calling zil_commit.

The attached patch fixes this issue.

Can anybody confirm this?

-- 
Alexander Zagrebin