Btrfs: fix number of transaction units required to create symlink

[ Upstream commit 9269d12b2d57d9e3d13036bb750762d1110d425c ]

We weren't accounting for the insertion of an inline extent item for the
symlink inode nor that we need to update the parent inode item (through
the call to btrfs_add_nondir()). So fix this by including two more
transaction units.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>