Caculate header size correctly in sa_find_sizes()

In the case where a variable-sized SA overlaps the spill block pointer and
a new variable-sized SA is being added, the header size was improperly
calculated to include the to-be-moved SA.  This problem could be
reproduced when xattr=sa enabled as follows:

	ln -s $(perl -e 'print "x" x 120') blah
	setfattr -n security.selinux -v blahblah -h blah

The symlink is large enough to interfere with the spill block pointer and
has a typical SA registration as follows (shown in modified "zdb -dddd"
<SA attr layout obj> format):

	[ ... ZPL_DACL_COUNT ZPL_DACL_ACES ZPL_SYMLINK ]

Adding the SA xattr will attempt to extend the registration to:

	[ ... ZPL_DACL_COUNT ZPL_DACL_ACES ZPL_SYMLINK ZPL_DXATTR ]

but since the ZPL_SYMLINK SA interferes with the spill block pointer, it
must also be moved to the spill block which will have a registration of:

	[ ZPL_SYMLINK ZPL_DXATTR ]

This commit updates extra_hdrsize when this condition occurs, allowing
hdrsize to be subsequently decreased appropriately.