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 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 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 of:

	[ 5  6  4  12  13  7  11  0  1  2  3  8  16  19  17 ]

Adding the SA xattr will, effectively require a registration of:

	[ 5  6  4  12  13  7  11  0  1  2  3  8  16  19  17  20 ]

Since the symlink (17) interferes with the spill block pointer, it must
also be moved to the spill block which should have a registration of:

	[ 17 20 ]

This patch simply re-uses the logic within sa_find_sizes() in which the
"split point" is calculated (near the comment "find index of where spill
*could* occur" to update extra_hdrsize in this particular case.